Our bathroom shower mixer operates via a bar with on/off water pressure switch on the left (press in black button and rotate housing) and temperature control via a press in and rotate red button on the right.

Recently the red temperature button has become stuck and will not move. Nor can I rotate the housing round it to choose the temperature. It seems to be a fairly common problem in hard water areas when you leave the temperature at the same setting for any length of time.

Can anyone recommend a solution? (The shower is around 3 years old).