I though it was also to do with them being un 'connected' with the right people.
I.e they were too small players to get away with it ,they were either going to get sold out (which they did) or the money was going to get taken off them by a bigger fish.
Nope, the main protagonist shall we say was very much the 'right' people, it's not so much someone stepped forward and 'assisted' old bill with their investigations but they (police) had them bang to rights anyway and when confronted with the sort of evidence they had (DNA etc), some of the people involved were 'forthcoming' and some weren't.
Either way, if the clear up had been as well executed as the job itself then they stood a chance of getting away with it.
The police would have known full well it would have required an inside man to help orgainse that job and that was always gonna be the weak link, if that held up then they might have gotten away with it.