Aqualisa shower valve servicing?

Try the model number and 'repair' and 'cartridge' in a search engine. There is a good market in repair kits, and many brands of shower use the same cartridge. You may also find that cartridge for a lot less than the shower manufacturer is asking.

It's a bit of a faff, but not much more than replacing the whole thing. You usually get a set of rubber washers/o-rings and some silicone grease. If you can get the kit, you will find instructions around the web. Obviously clean any limescale out. You'll find the rubber has hardened, which usually leads to difficulty it turning off completely, but sometimes other problems.

Some kits include a thermostat, if not they are available separately. You won't actually know if the thermostat is any good until you've done the other jobs and reassembled the thing, so given that you know you have a thermostat problem and not just worn washers, it's probably worth getting one and doing the job in one go. It's a moving part, so it may just need descaling and greasing, but it may not.

Lots of photographs, with all the parts spread out, you don't want to rely on memory for getting them back together.

I have two showers which look completely different, but in fact use the same cartridge, so I have a complete spare unit which I can refurbish at leisure and have it ready to swap in when necessary. I'd say they need doing about every 10-20 years.

A compatible cartridge is available for around £100 less but it still seems like too much to pay when a whole new fixture might make more sense.

I just need a “round tuit” and possibly a couple of stop-ends to allow me to turn the water back on whilst I’m descaling.

Yeah, probably overdue then. ;-)

Tim