Originally Posted by Richard

The boiler will start up as normal and run for around 5 minutes

After 5 minutes the burner will cut out and the gas flow is cut off (a quick
observation of the gas meter numbers shows this to be the case).

Since the overheat light isn't coming on and all the radiators get nice and
warm that seems to rule the pump out.

After the cutout the fan keeps running and the electronic ignition keeps
trying to reignite the system (to no effect as gas isn't been supplied).
The system then fails to do anything else for the "run time" set on the
timer.
-
Richard