Hot water problems from combi boiler?

No hot water in kitchen although hot and cold pressure is very good. However if I turn the hot tap on in the kitchen then go upstairs and open the hot tap in the bathroom, the boiler fires up and then I can turn off the bathroom tap, but continue to get hot water in the kitchen until the kitchen tap is turned off again. The bath also works fine. Any ideas what is wrong?

The Coachman
November 2009