I think the smoking gun, apart from anything else, is the fact that Diana contacted Tecwen - a vague acquaintance of an acquaintance whom even her husband claimed not to know of - FOUR times soon beforehand, and then he happened to be on the same show.
There's also the fact that, of all the potential contestants who might be inclined to plan in-depth to exploit all loopholes in order to gain maximum advantage - and then be tempted to nudge that into cheating and acting illegally - you would probably expect that the one who had co-authored a book on how to win might be a strong contender!
Then again, when the national lottery was relatively new, there were endless articles on how to win it and - most ridiculously of all - deranged idiots researchers advising on the 'hot' and 'cold' numbers that were more or less likely to come up again based on past experience. Either people who didn't understand the concept of a random lottery or tabloid hacks who were hoping that their readers didn't!
That said, I wonder if their rules expressly stated that the contestant wasn't allowed any third-party help outside of lifelines. Obviously, it's against the spirit and morals of the game, but legally, had they actually categorically stated this? I know they had a criminal-history clause and one big winner was later stripped of his prize when it emerged that he had a criminal record; but did they explicitly forbid what he apparently did in their T&Cs?
As Charles claimed, with the Craig David question, he had exploited the audience's gasps when he was apparently going to go for the wrong answer, and nobody on the show had picked up on this. Presumably, if an audience member simply shouted out the answer, they would stop filming, eject that person and then replace it with another question. They didn't raise any objection to the collective gasp of the audience (which could equally have been because he was going for the wrong answer or because he changed his mind so abruptly) - so where would 'ambiguous' coughing come on that scale?