A friend of mine is having the same problem. No hot water unless the central heating is on but even further, she says that even when she gets the system up and running, she hears a 'click' within 10 minutes of switching back to water heating only.
I remember my mom's old floorstanding potterton. We had that for 15yrs that I knew of with no major problems. Now that's good old fashioned British engineering for you!
What ever happened to those days? - Don't answer the last part just tell me about the biler if poss...