Hi , it mayv be locking out on overheat. Check pump running, bleed system especially manual air release valves on the pipe work. If these are ok, four be a thermostat issue, does the boiler attempt to relight, ie sparking .
Possibly a thermostat issue.might not be allowing the boiler to reach full temp. If not that could be a pcb fault
Is there any evidence of overheating,i.e. settling, red hot pipes?
Can we continue late, sorry but gave to go out for a few hours.
If boiler has air pressure switch check its making.
It may have a flue stat
Hi , the fault may be with the boiler thermostat. I would change the stat.
Ok, I'm out of suggestions ,it's pretty hard not being in front of the boiler. I would replace the photocell, just to rule that out