Afew months after David Thomas bought Le Fort de Bassens, a dilapidated chateau with 44 acres of farmland near Carcassonne, he met a man in the drive. “What are you doing here?” asked the stranger. “This is my house.”

“No, it isn’t,” retorted Thomas, who had just handed over about £250,000 for it, and started some expensive renovations. “It’s mine. So what are you doing here?”

“I’ve come to inspect my property.” “Have you paid for it?” “That’s not important.” This surreal encounter, in 2000, was the beginning of a seven-year nightmare in which Thomas, 63, a farmer and businessman, and his wife, Irene, 39, have struggled to prove by all legal means that their purchase was valid. They failed.

Their case has ended up in court no fewer than six times. The final judgment, made in March this year, went in favour of the man in the drive: Philippe Lheriau, an antiques dealer, was declared the rightful buyer.

Although Lheriau has yet to complete the transaction, the local bailiff has a warrant to evict the British rivals. For the past month, the Thomases have not known from one minute to the next when the axe would fall. “A bailiff arrived one day in a somewhat overexcited state,” Irene says. “He shouted at us, saying he wanted us out immediately.”

The couple have had huge support from other farmers in the locality, who threatened to turn up en bloc to protest at the event. “We are very angry,” one neighbour said. “Imagine, the eviction of a farmer in favour of a furniture dealer.”

It didn’t help. Last Wednesday came the final blow. The couple had asked the enforcement judge in Carcassonne to postpone their eviction until the end of July, to give them more time to arrange their affairs. “What we got instead was a slap on the wrist,” Thomas says.

“We were told to get out immediately and not to ask silly questions. We are permitted to harvest our wheat, but that is it.”

The couple, who are already packed up and staying with friends, are entitled to receive back the purchase price, but not what they spent on renovating the property – the same amount again. “We are required to hand it back as it was in 1999, so we have removed as many of the new fittings as we could,” Thomas says. “I’m damned if I’m going to leave anything more than I have to.” That won’t stop the couple ending up substantially out of pocket – the chateau is now valued at £1m.

For the moment, Le Fort, once witness to the genocidal tragedy of the crusade against the Cathars, is empty. And, with the identity of the rightful owner disputed, it is again in the line of fire.

So, how exactly did the couple get into this mess? “I had sold a wine property in Bordeaux,” Thomas explains, “and was seeking a farm and a family retreat in the Languedoc.” When he came across the chateau, which dates from the 12th century, in the village of Saint-Martin-Lalande, about nine miles from Carcassonne, it was love at first sight.

The castle, which has 15 bedrooms, had been on the market for about three years. Yet, suddenly, there were two bidders, at exactly the same time, and for the same money: Thomas and Lheriau. Inevitably, there was a flurry of fax and telephone activity between the owners, two estate agents and three notaires. Each side contends that its offer was accepted on the same day, November 15, 1999.

The day before, the sellers, Jean Du Lac and his wife, Joëlle, had sent a fax to Lheriau’s agent, Annick Gilles of the Hamilton estate agency, based in Carcassonne, stating that they were “disposed to accept” – but did not formally accept – her client’s offer. However, they wanted the details confirmed in writing and a deposit of about £50,000 paid to their notaire.

The Du Lacs didn’t hear back, so, the next day, they called Thomas and told him Le Fort was his. At the same time, Gilles took Lheriau into the office of a different notaire, where, unbeknown to the Du Lacs, he signed a unilateral compromis de vente (promise to buy) and handed over the deposit cheque.

It was this piece of paper that swung the latest case. Although the Thomases’ offer had been officially accepted by the vendors, the court judged Lheriau’s compromis de vente to be valid. This was despite the Du Lacs’ repudiation of it – and despite the fact that the sale had later been completed through a notaire appointed by the Thomases.

Too many notaires? Too many bits of paper? Many lawyers now feel that the use of formal “offers to buy” or “offers to sell”, especially by estate agents, makes for a confusing and dangerous extra layer of paperwork in any French property transaction. “It is a minefield,” says one notaire not involved in this case. “If things get complicated, walk away with your pen – and your money – in your pocket.”

Gilles takes a different view. “The French law is very good and very clear,” she says. “Our success in this case proves it.” (Her clients, who were not available for comment, are naturally delighted with the result.)

Still thinking of buying in France?

Times recommends

• Second homes: credit crunch casualty or money-spinner?
• How I built my own sliding house
• Top 10 incentives for house buyers