Tepid Output

Yes you should be able to service this from the front OK, it may be a bit tricky getting the cartridge out but not impossible.

With the Sirrus 1500 or 1800 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

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.

The seal housing is available which has the washer already pressed in if for some reason you have difficulty fitting the new seal.

The 1800

http://www.showerdoc.com/shower-spares/sirrus-by-gummers/GUM-PARENT-14-Sirrus-1850---1850-Dual-Shower-Valve

There is no real difficulty with this valve however it is a little more complicated than the 1500 just take photos as you strip it down you can then refer to them when you are rebuilding.