Likely issue here, is the Thermostatic Cartridge has failed. This can happen over time and especially so, where a PEV ( Pressure Equalising Valve was not fitted) during the install. An imbalance in pressure between the hot and cold over time, can damage the thermostatic cartridge and gradually the temperature will not maintain as it should.
To replace the thermostatic cartridge, head over to www.steamshowerparts.co.uk where you will find a guide on how to remove and identify, which cartridge is required for your steam shower, shower cabin or whirlpool shower.
Note these are not specific to model, so you will need to remove the old one in the first instance and compare this to the ones displayed on the parts website.