I've worked Monday to Friday for nearly twelve years and now they are expecting me to work on Saturdays - where do I stand?

[Osirus]

I work in an office in a field where we traditionally work Monday to Friday, 9-5pm. I've worked here for almost twelve years and they are shortly going to be closing the branch I work in and opening a new one. They have asked us to also now work Saturdays on a rota-basis at this new branch (the other branches don't open Saturdays).

I don't have a copy of my contract, but I'm sure it didn't mention weekend working. I am happy to do occasional weekend working, I always have (I worked yesterday for example, and didn't claim overtime). I don't however want to be tied into set weekend working.

After twelve years working Mon-Fri, can they insist I now also work Saturdays?