For batteries use the Wein cells referenced above from Small Battery co. They fit without physical adjustment of the battery compartment and deliver the correct voltage. They don't last as long as the original mercury but you don't need any other adjustment. Luckily I have original mercury batteries still going strong.
If you ever need a service of these cameras I can highly recommend Ed Trzoska Leicester in Leicester on 0116 2674247 who did a great job on mine cleaning the shutter blades properly, and checking, cleaning & calibrating everything else, it's like new.