Combi Boiler wont allow me to turn heating off?

I have a worcester 28cdi Combi boiler that is constantly switched on allowing me instant hot water at the turn of a tap!! recently we woke early hours as the heating was on and have found out now that we cant turn it off unless we unplug it...
This now means every time we switch the boiler on so we can get some hot water the heating automatically comes on also... I have tried the reset button but it doesnt seem to reset it at all!!! HELP.....