How can I fix my Glowwarm combi heating. It keeps shutting down?

I have a glowwarm compact 100e combi boiler (not sure of age - inherited it with house). The problem is:
Within 1 hr after the heating starts the boiler shuts down. The overheating warning light comes on and the boiler is indeed quite hot. The water pressure rises from 0 (does not remain at the set 1.3 bar, and always returns to 0 when heating is off) to it's max then shuts down. Once the heating has been switched off for a couple of seconds the warning light disappears and it powers up again when switched back on.

Can anyone help?