Ian
This sounds like it may be a thermostat issue with your shower. Is is still under warranty? I know this sounds silly but have you read the manual? how hot is the water in your tank? did you fit it or was it a plumber? maybe he can help?
is it meant to be run from mains cold or tank cold and have you got this right? are you sure? if its not right the cold water will force past the thermostat and cause your problems and may be more inclined to do so when the shower has just been used as the internal workings of the stat will be in a different position relative to when its cold.
Kenny F
December 2008