I have a concealed valve shower, pump in the attic. Gas boiler heats the water. Over the summer the radiators were turned off. Since I turned back on the radiators the water in the shower is not hot - its hot for 30 seconds when the shower is turned on but then goes cold. Hot water in all other taps. Had the same problem last winter but water not as cold. The water in the shower has never been as hot as taps but is ok once radiators not turned on. No luck getting a plumber last year. Don't suppose anyone can suggest a simple solution to save me trying to track down a plumber?