Fight to own Britain's biggest mega-mansion: An oligarch whose daughter-in-law is 'too pretty to work' is trying to see off an ex-colleague who says a verbal agreement entitles him to £300m pad in London. DOMINIC MIDGLEY explains the very ugly court battle

When Britain's biggest private house, a mega-mansion called Witanhurst on Highgate Hill in north London, went on the market in 2008, it was close to a ruin.

The roof was riddled with leaks, the floorboards were warped and cornices embellished with gold leaf were blighted by mould.

Given that this 90,000 sq ft property had a total of 65 rooms - including 25 bedrooms - and a ballroom 70ft long and 20ft high, with an elaborate ceiling that takes six months to paint, it amounted to the country's ultimate doer-upper.

The enormity of the task ahead put off a string of would-be buyers, until one with the vision to see past the devastation - and with pockets deep enough to fund the process - stumped up the £50million asking price.

His name was Andrey Guriev. Over the next decade or so, the Russian fertiliser billionaire presided over a massive renovation, which - at its peak - involved an army of decorators, plumbers, carpenters and landscape gardeners burning through £2million a week.

The old service wing was demolished and replaced with a three-storey villa called The Orangery and five and a half acres of ornamental gardens were landscaped.

But the most ambitious element of the project was the construction of what is reputed to be the biggest basement ever built in London.

At 40,000 square feet, it houses a swimming pool, sauna, massage rooms, a gym, a cinema, staff quarters and parking spaces for 25 cars.

But Guriev, 64, had precious little time to enjoy his British dacha. In 2022, shortly after Russia went to war with Ukraine, he was sanctioned by the West and issued with a travel ban thanks to his status as a crony of Vladimir Putin.

Now he faces the possibility of losing Witanhurst altogether. On April 15, a case listed as Gorbachev v Guriev began at London's Commercial Court.

Alexander Gorbachev (no relation to the late Soviet leader), a former director of Guriev's company PhosAgro, is suing him for £2.7billion: the 25 per cent stake in PhosAgro that he claims he is entitled to, plus the value of the dividends he has missed out on since the company was floated in 2011.

In the normal course of events, the two men would be exchanging cold stares on a daily basis in Court 19 at the Rolls Building, a modern, purpose-built courthouse in the heart of London.

But thanks to his travel ban, Guriev is unable to appear in London and so the presiding judge, His Honour Mark Pelling - accompanied by the warring legal teams - will fly to the UAE this coming week to take his evidence over four days on neutral territory at the Dubai International Finance Centre Court.

It's a sign of how attached Guriev is to his vast London home that he is cooperating with the process at all. After all, the oligarch - who is worth an estimated £7.5billion - could quite easily ignore the British courts and refuse to pay any penalty they judged him to be liable for.

In that case, however, his foreign assets could be seized, and they include Witanhurst - acquired via an off-shore company called Safran Holdings registered in the British Virgin Islands - which is now valued at more than £300million.

Not a bad capital appreciation for a property that was built in 1913 by the Liberal MP and soap magnate Sir Arthur Crosfield on an 11-acre plot for £1million (£37million in today's money).

Witanhurst means 'Parliament on the hill' in Anglo-Saxon and, in addition to a glass rotunda, it has 365 windows – one for each day of the year.

Sadly, Sir Arthur did not have long to enjoy the charms of this great edifice. He lost a fortune in a failed Greek mining venture and died shortly afterwards on a train journey through southern France after falling out of the window of a sleeping car.

While Witanhurst is the jewel in Guriev's Western assets, others that may have caught 61-year-old Alexander Gorbachev's eye are a £7million 'triplex apartment' overlooking the Thames and a 267ft superyacht called Alfa Nero that lies abandoned in Antigua.

If he is to wrest these from his former colleague's grasp, he must first persuade Judge Pelling that he is indeed the rightful owner of a quarter share in PhosAgro. Unfortunately for him, however, his case - in the absence of a formal contract - relies on proving that they had a verbal agreement, reached during a series of meetings in 2005 at locations including a London pub and a sauna.

Such an arrangement was not unusual when the two men first went into business together in the mid-1990s. In those days, when President Boris Yeltsin sold off hundreds of billions of pounds worth of state assets in a series of sweetheart deals with unscrupulous opportunists, Russia was known as 'the Wild East' and many deals were done on little more than a handshake.

In the course of researching my biography of Chelsea FC's former owner Roman Abramovich 20 years ago, I discovered that, when he and his business partner Boris Berezovsky acquired the oil giant Sibneft, they did so with nothing in writing to say how the shares were to be divided.

It later emerged that while Berezovsky saw himself as a 50-50 partner, Abramovich considered him to be nothing more than a top-level Kremlin fixer, who he had showered with gifts, such as yachts, planes and real estate, and paid him 'more than $2.5billion'.

Their dispute spilled over into the UK courts in 2012 when, after the biggest private litigation battle in British history, a judge threw out Berezovsky's claim that the two had an 'oral agreement' that they were equal partners.

Like Boris and Roman, Gorbachev and Guriev were something of an odd couple from the start. While Gorbachev was gregarious and personable, Guriev was an introvert who had married his childhood sweetheart and, at one point, lived in the same modest apartment building as his driver.

Gorbachev's troubles date from the moment Vladimir Putin turned against Russia's richest man, the oil oligarch Mikhail Khodorkovsky, who was a 50 per cent shareholder in PhosAgro. In 2003, Khodorkovsky was arrested, charged with fraud and ended up in a prison camp near the Chinese border. Gorbachev, fearful of being targeted by association, fled for London.

For years, Guriev kept Gorbachev happy by making payments to an offshore company opened in his name. But when PhosAgro was floated on the London Stock Exchange in 2011, Gorbachev found to his consternation that his name was not on the list of shareholders.

When Guriev tried to fob him off by offering to buy him out for £30million, Gorbachev angrily refused the offer.

The current action dates back to October 2018 when Gorbachev had Guriev served with legal papers while shopping on Berkeley Street in Mayfair.

A video of the incident appears to show bodyguards attempting to shield Guriev from the 'process server' and the papers were eventually given to one of his assistants.

Guriev argued that this meant he had not been properly served but at a court hearing in Manchester in 2019, a judge ruled that the assistant was an appropriate proxy and proceedings continued.

Gorbachev is not the first disgruntled PhosAgro employee to seek satisfaction via the British courts.

Igor Sychev, a tax lawyer, says the company promised him a substantial bonus amounting to 1 per cent of the company's worth after he negotiated a massive tax discount with the Russian authorities but Guriev failed to pay up.

When he embarked on legal action, however, Sychev claims he became the subject of 'assassination attempts' by Putin cronies, and his enemies 'bribed his lawyers' and 'threatened to murder his wife and children'.

'The Putin-friendly oligarchs repeatedly threatened to kill me, demanding that I drop this lawsuit,' he told MailOnline early last year.

Sychev also claimed his car was sabotaged three times in an attempt to kill him. 'The cars were new and expensive Jeeps and in the first case everything happened quickly at high speed on the highway.

'According to the woman driving behind me, she saw that sparks of fire suddenly flew out from under the wheels of my car and immediately after that the front wheel with the accompanying iron fasteners came off. This huge wheel flew at this woman's car and she miraculously dodged it.

'My car crashed into a truck moving parallel and turned into a pile of scrap metal that cannot be restored. I survived thanks to the airbags.

'The second incident was two weeks later, also at high speed on the motorway, which was quite empty at the time.

'I felt that the car began to behave strangely and it almost did not listen to the steering wheel. I stepped on the brakes but the brakes didn't work.

'I was able to stop only by inertia after about two minutes and only by happy chance avoided colliding with cars or people. Video shows that sparks of fire were flying from under the car.'

He added: 'The third incident occurred a month later and then the wheel also fell off, but this time it happened at a low speed when approaching a traffic light. After that, I crashed into a parallel car, but because of the low speed, no one was injured.

'It was incredibly difficult for me to realise that the people for I worked for for 18 years wanted to kill me.'

A spokesman for PhosAgro described the alllegations as 'wild and unsubstantiated'.

While Sychev is happy to speak out, Guriev has never given an interview.

Biographical details are sketchy but he is believed to have grown up in Lobnya, a town near Moscow, where he was a teenage martial arts champion, a passion he shared with judo-loving Putin.

Guriev and his wife Evgenia went to the same high school and are reported to live in a mansion in Forest, a gated community on the outskirts of Moscow that is popular with oligarchs.

Unlike her low-profile husband, she is said to have a taste for the high life and owns a stable of classic and luxury cars, including two Rolls-Royces that once belonged to Elton John.

But it is their daughter-in-law Valeria who appears to have embraced the trappings of wealth with the most gusto.

She once used a photograph of herself on a powerful motorcycle in a short black skirt as the banner image on her Facebook account. Other images on social media showed her swigging champagne from the bottle on a yacht and posing with a revolver strapped to her bare leg.

According to the New Yorker magazine, she also documented trips to the Bolshoi, Monte Carlo night club Twiga and the Michelin-starred restaurant Per Se in New York.

However, she turned her accounts private a few years ago after she became the subject of an online pile-on over her Instagram slogan worthy of Marie Antoinette: 'I'm too pretty to work.'

The same cannot be said of her plump, balding husband Andrey Guriev Jr, who stepped down as CEO of PhosAgro in 2022.

The Gorbachev v Guriev is scheduled to last for six weeks and so, by the beginning of June, Gorbachev will know whether he is destined to be one of life's embittered also-rans or the proud owner of the biggest house in Britain after Buckingham Palace.

Dominic Midgley is the author of Abramovich: The Billionaire From Nowhere