What is causing my boiler to randomly operate the central heating?

My boiler, a Potterton Puma 80e, works fine on the hot water cycle, but randomly operates the central heating.
The heating packed up for a few days last week then one evening when I got home, all the radiators were hot and the heating was operating normally. this continued for a few more days then guess what? The heating was off one cold morning, but the water works fine.
Today, it seems, if I let the hot water run for a while, say filling a bath, the heating switches in once the tap is turned off.
No taps are dripping, the thermostat seems to be operating normally.
What could be wrong?