New cartridge solved my cool showers
Was about to replace the entire unit but decided to try a new cartridge.
Process is straight forward but takes time.
Removing old cartridge (brass version) was difficult,
I had to take whole unit off and put in a lagged vice to get it unscrewed.
fitting new unit easy enough (lubricant supplied).
Re-fitting temperature restrictor ring very hit / miss - gave up after 40 mins
Good video on site