Please see below for instruction on calibrating a thermostatic cartridge.
How to change the temperature on a thermostatic valve
Turn the water off at the mains
On the back of the tap head there is a cover cap remove this.
Remove the 2ml grub screw with a 2ml Allen key.
Remove the chrome collar with a Phillips screw in.
Remove the shroud which is on a thread.
Remove the nut holding the cartridge in place.
Then remove the cartridge by turning this.
To recalibrate follow steps 1-4 and the pop the collar back on and turn the cartridge all the way to the right and then back to the left adding hot as you go, preferably with the shower or bath on, when you get the required maximum temperature stop and put everything back on.
How to clean a Thermostat
Follow the steps above. Once you have removed the cartridge, you need to soak this in 50% white vinegar and 50% water. Leave immersed in this solutions as long as possible. Then replace this back in the valve the longer you leave this the more lime scale debris will be removed.