what could be wrong with my boiler?

I have a glow worm compact electronic 100.The central heating works fine but but hot water is a hit and miss. However if two hot water taps are open at once , then hot water is available.One tap can then be closed . If the second tap is closed and then reopen, hot water is not available.The plunger operating the hotwater demand switch was found to to be leaking a little amount of water, not enough to stop the switch operating when two hot water taps are opened.