Not hot

Regarding the temperaturefluctuation or cool running it could be a faulty cartridge or the cartridgeneeding serviced however to check this, see if you can get hold of the hot supplypipe before it enters the shower or perhaps as it leaves the boiler.

If when holding this pipe thetemperature fluctuates or goes cool then the problem is not with the shower butmore likely with the boiler as the shower can only deliver what the boilersends to it.

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

If the incoming temperature issteady then the valve will need a service

The 1500 / Cardinal

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

The valve looks complicated however it is really just a caseog removing each seal one at a time matching it with the one in the seal kitand fitting the new seal

The seal that shuts the water off is theseal that fits into the base of part 5 in an old valve this looks like a flatwasher, 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 sealis removed and no bits are hiding under the lip, give the new seal a heat inhot water and make sure that the seal is well greased before fitting, once itis pressed in there can be bumpy bits on the face of the seal, rolling andpressing a screwdriver handle over the bumps a few times can help to smooththem out.

The seal housing is available as a sparewhich has the washer already pressed in, if for some reason you have difficultyfitting the new seal.

http://www.showerdoc.com/shower-spares/sirrus-by-gummers/GUM-SK1500-22-Sirrus-Ts1500-Housing-Kit-SK1500-22

Take photos as you strip it down you canthen refer to them when you are rebuilding.