If your shower is unable to provide hot water, even with the dial set to the hottest setting, this would indicate that the Thermostatic Cartridge has failed.


You will need a replacement Thermostatic Cartridge. These are not model specific so first you will need to remove the existing one that has failed, and identify which type you have fitted. 


Visit the SSP parts website at the URL below and compare the existing one to the images on the web page. There are only three main types for steam showers and cabins - Type 6 [this is a push in one with small grub screw] Type 8 [a screw in one] and Type 12.


http://www.steamshowerparts.co.uk/product-category/thermostatic-cartridges/