Brittany bar mixer valve fluctuations in temperature

[stratplus]

Shower valve off combi boiler goes hot/cold and is never constant.
Does anyone ever “service” these or is it quicker and cheaper to replace whole thermostatic valve?
Can’t imagine there’s much to them apart from the cartridge and inlet filters.

 

[king of pipes]

Cheaper to swap for a new unit just check it is the shower and not dirt in the domestic heat x causing the boiler to cut in and out while providing hot water ? . Kop