One of the hardest accusations to prove is participation in a deal whose terms have never been openly discussed or documented – when it is all winks and nods. So when David Cameron insisted in Parliament on Monday that the Conservative Party had never had a "grand deal" with Rupert Murdoch over the the BSkyB bid, he couldn't be gainsaid. Under pressure, the Prime Minister would have been able to say – show me the facts that support your suspicions. None would have been forthcoming.

I had this in the back of my mind this week when I read extracts from transcripts of an interrogation of Dominique Strauss-Kahn by French magistrates. For they were engaged in the same sort of hopeless task, proving an implicit conspiracy, only in this case they were trying to implicate the former head of the IMF in arrangements involving prostitutes. These enquiries are quite separate from the criminal charges laid against Mr Strauss-Kahn in New York, of sexually assaulting a hotel maid, that were later dropped. The French case involves a suspected prostitution ring centred on the Hotel Carlton in Lille.

The account is as gripping to read as a "police procedural" – the type of crime fiction that gives a minute description of the activities of the police as they go about their business. More than that, though, DSK gave a masterclass in rebutting the allegations.

The plotline is that four friends, comprising two businessmen, a police commissioner and the son of a local politician, all from north-east France where DSK began his political career, arranged parties for him where the only other guests, apart from themselves, were young women available for sex. You can see what was in it for the four friends. They would give DSK a good time and, when he became President of France, which was an excellent bet until the New York incident, they would have their rewards. And no finger of suspicion would be directed at anybody.

The magistrates tacked this way and that trying to find a favourable wind. They began by saying that the four men had arranged 15 parties for DSK's benefit in Paris, Lille, Belgium and Washington. "It was never apparent to me that they were for my benefit," DSK responded. "But one of the friends states that while you were not the instigator, you were obviously happy to be involved." DSK replied: "He is right to say that I was not the instigator, but I am always happy when my friends invite me to something."

Then the magistrates turned to something one of the girls had said. DSK was very active so far as the sex was concerned and he was especially interested in newcomers rather than the more familiar girls. "As to the first part," commented DSK, "that is her opinion. And it is true that the interest of these occasions rests in part upon their novelty."

Another witness had told the magistrates that the girls at these parties were dressed in a provocative, vulgar fashion. Again DSK accepted the facts. "At these types of evenings, the girls can be provocatively dressed, but that doesn't prove that they were prostitutes." Then he offered the magistrates a lesson in debauchery (le libertinage is the word DSK used). "You must understand that it consists in having free and consensual relationships. You may think what you want so far as morals are concerned, but the sex was not paid for."

But what about the differences in age? Two of the girls were in their early twenties. "Do you think that if they had not been paid, they would have contemplated having sexual relations with you taking into account the age difference?" "Madame," replied DSK to one of the magistrates, "many of the young women with whom I have been able to have a sexual transaction – sorry, the word is badly chosen – a sexual relationship, had the same difference of age."

Finally, the magistrates are getting somewhere. DSK has moved away from his "accept the facts, dispute the interpretation" line and failed to prevent a slip of the tongue – "a transaction". And, at the same time, he has become boastful about his conquests, potentially another weakness. So they press DSK further on the use of language.

In the messages between DSK and the four friends that the magistrates had seen, he had referred to the girls as "material" and, when the friends and the girls had had to travel to meet him, as their "baggage". The magistrates commented: "You seem to have little regard or respect for the young women, one of whom has told us that she felt as if she was an object for purely sexual consumption." All DSK could say was that he had indeed used such descriptions but that he did not, nevertheless, consider women as objects.

The masterclass is beginning to go less well. But, unfortunately, the extracts stop at that point. I doubt if the verbal slips will serve to convict DSK. You cannot prove "understandings". Only agreements will do.

a.whittamsmith@independent.co.uk