New thermostatic shower cylinder fitted - now water is not hot enough

[BrianCox]

I have a Bristan mixer shower. It started leaking from the temperature control. A local plumber replaced the thermostatic cylinder. It now doesn't leak - but the water is now 15-20 per cent colder than it was before when the temperature is at max hot. Why? What's amiss here?

 

[SimonG]

Adjust cartridge and check filters.