CH Boiler problems

[RWA28]

Hopefully there's some heating engineers on here tonight!
My parents came back off holiday yesterday and we can't get the boiler to work. (been switched off for 2 weeks)

It's an oil fired boiler (not a combi) and we're just getting the lock out light coming on all the time. Here's what happened:-

Turned boiler back on on Tuesday night
Went back Wednesday and lock out light was on, I thought they'd run out of oil but tank was full. Reset the lockout button and it tried to start up then tripped the breaker in the house fuse board!!!
Tried it again and it tripped again.

So today we've fitted a new control box, new ignition transformer, new capacitor, cleaned the flame detector cell, bled the fuel through to check for air and fitted a new nozzle.

Doing all this has now stopped tripping the breaker and it fires up. The problem is it fires up then after 10 seconds or so goes to lock out again.

There doesn't seem to be much else we can change other than maybe the fuel pump?

Any help or suggestions would be greatly appreciated.

[RWA28]

Don't think it's anything to do with overheating water as it doesn't get that far.
Could be the photo cell as that would cause it to shut down if it thinks it isn't seeing a flame. Will try that tomorrow, I'm thinking that the pump is ok as the flame is fine when it does come on for the 10 secs or so.
Thanks for all the suggestions chaps!