worto03:
not sure if it being a business account would make any diference but if it was my account I'd leave it in there for a couple of months then transfer it out to another bank, I still wouldn't spend it but you should be able to get about £50 a month interest off it until they notice.
If they don't notice after a few years then I'd spend it. Personanly I'd wait about 6 years thought before spending it but that just depends how hard you'd find it to replace if they came back for it after you'd spent it.
worto.
I think this is about right, something like 6 years until it becomes yours. Sit on it and don't spend it. I bet they'll take it back though.