Mira 722 mixer tap - not mixing

Hi, anybody got any advice? I have a Mira 722 shower and the mixer control does not have any effect on the water temperature. Used to work occasionally in that it would flip between red hot and stone cold. So I'm guessing its the mixer itself rather than the thermostat. Any advice?
Do I try the service pack first, then the mixer part, then the thermostat.

Also, struggling to actually get to the mixer control i.e. removing the water control knob. The whole thing is fitted flush inside tiled shower surround.

Thanks in advance

The coil will be broken inside and need replaced, this regulated the temperature of the water. You just need a bit of brute force to get into the mixer or apply a bit of heat first eg very hot cloth or heat gun carefully. If you are taking out the insides you might aswell replace the o-rings too. There is a kit available from suppliers.

If the coil is removed would it act as a straight forward mixer?

Nope. Coil turns a barrel inside to regulate temp, if you remove coil, no connection between temp control and barrel.
You will be able to get instructions online or if you order a seal pack. Also seeing it is such an old post you should post a new thread to save digging up a 15 year post, Can't believe been on here that long. LOL