Worcester Heatslave Combi 15/19 boiler keeps cutting out?

Hi

I'm hoping someone can help solve the mystery as to why my boiler keeps cutting out.
The problems started when it went into lockout mode. Turns out we were just about out of oil and have since had the tank refilled.
Since then, it leeps locking out and whenever we hit the reset button :
we get 15 seconds pre-ignition,
the boiler ignites,
it burns for approx 2-5 minutes before locking out again.

Sofar we've . . .
-bled the oil pump
-repressurised the expansion vessel
-checked electrical connections on the control box
-cleaned the photocell
-checked and reset air intake.

Any ideas as to what's causing the problem, and else we can do to fix it?

Any help would be greatly appreciated.

Thanks