Depends on how good you are at DIY.
Changing that whole bath/shower thermo tap isn't too hard if you have the correct size of adjustable spanner, are reasonably handy and can shut off your hot and cold water.
You can pick up a cheapo thermo shower filler tap from fleebay, shut off the water, and then loosen the 2 nuts, get rid of any PTFE tape from the threads (big no no) and then fit the new one, pipe gaps should be the same (150mm).