Can I still return these books if I'm out of the 21-Day Return-Back Guarantee?

I bought 2 books from Waterstones (book store in sunny UK) for an ignment but found them in the library instead, so never had to use them and left them on a shelf for a couple of weeks. In all my idiocy, I thought I had a 28 day return-back guarantee when it's actually a 21-day back thing. Tomorrow will be the 25th day. Do you reckon I can go to Waterstones anyway and try to return them, or will they either make me keep them or shush me with store credit?