I have always thought that he knew roughly how much because either he or NB had collected takings from the camp site (it was peak holiday season). He would know were his dad kept it. When someone dies their bank accounts are frozen until after probate. JB was probably advised to search out as much cash as possible.
Neither of them 'collected' takings from the camp site, unless you have a source stating that? That's what the office was for - the one Bamber robbed. Jeremy was involved in the harvest - you're just making stuff up to explain something there is no good reason for. You accuse others of doing something you do regularly yourself. There is NO good REASON why he would know how much was in the wallet and if there was, then he would have said so, instead of avoiding the question for two years! Instead of trying to think of reasons why he would know, why don't you explain why he didn't give the reason instead of being evasive because that's the POINT!
'JB' wasn't advised to search out cash at all - Basil Cock (the executor of the will) had already asked the Eaton's to do that because they had the key. Jeremy worked and was paid so he had wages to live on.
Even if he guessed that the wallet was most likely in his fathers pocket, there was no reason to believe that it was missing' just because it wasn't there. His father may have put it some place else but Jeremy seemed to KNOW it missing and the only way he would know that, is if he was the last person to see it there!