You bought the house - I would have thought by now if the previous owners were missing money, they'd have come back to you by now. It might not even belong to them - it could be people who lived there several owners ago, don't give it to the wrong people!
If you are going to keep it, do be prepared to be quizzed by the bank when you go to change it or deposit it. My mother had some money from an insurance policy which matured, and the money laundering clearances she faced in trying to open a new account with it were considerable. I'd suggest that you write to the Bank of England explaining exactly what happened, that nobody has come forward to claim it, and will they convert it for you. You have to tell them how you got it, otherwise it will look fishy from a criminal point of view - which this isn't.
I think I'll go look in MY loft....