Hello Nick,
I am not a legal expert, and I always recommend folks seek professional legal advice if they wish to pursue a claim. It is up to the individual if they take that advice or not.
It does not surprise me to read that your son-in-law believes that the French gites terms and conditions would not stand up in court, because it is his job to try and find loop holes or at least to make others believe there is a case to answer.
A barristers job is to challenge and effective convince you that black is white, but that does not necessarily make them right, only that the wining barrister has placed sufficient doubt in the minds of the judge/jury to change the perceived balance of guilt/innocence.
The view expressed by your son-in law about Gites documentation is a red herring. The documentation by your own statement was a standard French item and thus would have been developed under French not UK law.
I am keen to see that no one is unfairly disadvantaged, but at the same time I believe that it is beholden of anyone entering a contract to make the appropriate enquires and to seek clarifications before agreeing to a binding situation. That is a position that the Sale of Goods Act takes up when it recommends the buyer beware.
The point to Gareth's position is that if he was denied the opportunity to read the terms and conditions before agreeing to the contract then it may be void. If he was offered the opportunity but refused to take it, then that is his problem, not the sites.
Being a private enterprise matter, the content and method of the contract terms and conditions are not governed by any statutory regulations. This means that the site operator can operate any cancellation policy they like.
Of course anyone can challenge the terms of a contract, but just because you can, neither makes it necessary or right. And if the challenge is driven because a party did not make the necessary enquiries before agreeing, then the case is already weakened - unless the terms can be shown to be unfair.
Perhaps you son-in-law would like to join the forum and offer his processional advice?