dont know much about money and banks but just a thought...
why not put it in a high interest account and just leave it there for about five years then if they want it back u can just say that 'i put it aside so i didnt spend it' and you will have made a few pounds in the process to buy an ice cream on the way home from the bank. if they dont ask for it after five years you can safely say that they have forgotten about it and then you will have perhaps an extra 100 pounds on top of the original money.
Pete