Shower not working properly on baxi with calor gas?

I have a combi baxi which is run on calor gas. The central heating seems to work fine but the shower is now not working properly. When my partner showers he is usually lucky and has hot water. When its my turn it doesnt seem to work and the water runs cold even though there is sometimes at least a couple of hours between the two showers. Why is the boiler suddenly playing up?? The plumber who was called said it was the diverter which regulates the gas pressure. He changed this (at a considerable cost) but it has made no difference, and he is now saying that he only changed it in order to be able to diagnose the real fault. I feel he has not a clue about the cause. Any help would be appreciated.