He was an appalling actor, though (Charles Ingram, not Michael Sheen). I know he was very nervous (as of course you would be, even if you were playing it honestly), but if he'd played it cooler and burbled less, they quite likely would have got away with it.
I don't think they identified the coughing pattern until their suspicions were roused by his ridiculous "I think it's B. Yes, I'm pretty confident that it's B. I've never even heard of D. Actually, I'm going to go for D."
If he'd just said "Hmm, now I'm going to think about this logically, take my time and slowly consider all of the options" and then non-committally read out the four answers, waited for the cough and THEN, after a few seconds, started to discount the others, his rumbleability factor would have been massively reduced.
They also should have kept quiet about her brother having already gone on and taken away that extra angle of potential suspicion. I don't know if they would ever have flagged this up with the different surname. If they had later noticed and commented, he could always have lied and said "It's a sore point, he was lording it over us, then Diana only managed to equal him and I wanted to thrash him first before rubbing his nose in it, but didn't want to tempt fate" or something like that.