I don´t find it that strange that Jeremy knew approximately what was in Nevill´s wallet. He was at the farm every day working, knowing the routines of his father and the running of the farm in detail.
He would very likely know when Nevill went to his bank the last time to get cash. People are creatures of habit, Nevill would probably take out the same amount every time he went to the bank - my guess is that this amount was 500 pounds, then Jeremy would be able to chance a guess as to what was in the wallet compared to the time between last visit to the bank and the murders.
I can´t see how Jeremy, who broke into the caravan park and took cash would look into his father´s wallet that night and NOT take any. Doesn´t ring plausible that he would have left such a large amount in the wallet (how much was in the wallet exactly btw?)
I CAN see that it is strange that he didn´t answer Caroline properly - or more correctly, in two versions.
Hasn´t Aunt Agatha said about Jeremy that he lies constantly? She still thinks he is innocent, some people are habitual liars, perhaps Jeremy is one of them.