combi problems

[headman]

Hi, My combi boiler has stopped giving hot water on demand unless the heating is on. I assume that there is a pressure switch or flow valve but that must be working as it all works when heating is on. At first I had to turn on the hot tap to have a shower but now that does not work. As it is warm now the house is like the tropics just so that we can have hot water! Please help!