no hot water upstairs?

hiya i have a chaffatoex and maury combi and had no end of problems with it. Its been seviced by british gas and Eon and now no-one else will touch it and they wont insure it!!
When i put the hot tap on upstairs in the bathroom, no hot water comes through. The pressure seems fine tho but you cant hear the boiler start up when the hot tap is on. The only way to get it working is to put the bath tap on too. But then it goes off after a while and you have to go through the whole process again. The kitchen tap (which is next to the boiler) is fine and works fine. Please help... thanks claire