Central heating stops after about 2hrs running?

I have an Ideal Mexico Super RS 80 boiler which seems to run the heating system for about a couple of hours whether on constant or timed, then shuts off.

If the switch ("heating and water" or just "water") on the Danfoss 3020p programmer is flicked between the two settings, the boiler invaribly starts up. A new Danfoss Programmer has been fitted and the circulating pump is fairly new and runs freely.

When on water only the system seems ok. When the boiler is running turning the temp. control on the boiler front panel to 0 shuts off the boiler and restarts when turned to the 4 - 5 setting. Any ideas to resolve would be welcome. Many Thanks.