I'm local and I can probably do it for you! What's the code of the battery you need? (engraved on the battery). Let me know, and if I've got one (I have a selection of most common types, along with teeny weeny screwdrivers), you can pop over at five past nine to do the job.
I've changed the battery in several watches where you undo two or three tiny little screws and the back comes off, then there is a little clip holding the battery in. You're very welcome to call in on your way home (I'm between Borough and London Bridge), or I will be going near you later on and could call in, or you could find a proper person if thats easier for you. PM me if you want to give it a try.