Hot and cold water change temperature during shower

Regarding the temperature fluctuation or cool running, it could be a faulty cartridge. However to check this if you have the shower fed from a combi boiler, see if you can get hold of the hot supply pipe before it enters the shower or perhaps as it leaves the boiler.

If when holding this pipe the temperature fluctuates or goes cool, then the problem is not with the shower but more likely with the boiler, as the shower can only deliver what the boiler sends to it. 

Disregard the temperature at the taps as they have a different flow rate.

Any competent plumber should be able to fix this.

With the Sirrus 1500 or 1400 series that in almost every occasion the first thing to do is give them a strip down clean and lube and fit a service kit, this will cure 85% of any problems and it is also the cheapest thing to do.

If after servicing you find out that it needs a thermostat or piston, nothing is really lost because it almost certainly needed a service in any case.

The 1500

http://www.showerdoc.com/shower-spares/sirrus-by-gummers/GUM-PARENT-34-Sirrus-1500---Thermostatic-Shower

http://www.youtube.com/watch?v=4DFGHKrnCkQ

http://www.youtube.com/watch?v=tFu44LLh994

The seal that shuts the water off is the seal that fits into the base of part 5 in an old valve this looks like a flat washer, it is in fact a thick heavy ‘O’ ring,  this can be a difficult seal to fit.

When fitting make sure that all the old seal is removed and no bits are hiding under the lip, give the seal a heat in hot water and make sure that the seal is well greased before fitting, once it is pressed in there can be bumpy bits on the face of the seal, rolling and pressing a screwdriver handle over the bumps a few times can help to smooth them out.