It runs Ok on domestic hot water , but on central heating after the initial 1st run upto temperature the boilerstat shuts the flame down and then when it next calls for heat it goes to lockout. It will light up for a few moments each time but fail to hold the flame. It goes through this cycle several times before lockout,with the lockout light flashing slowly.The engineer has tried a new board to no avail.

Dave Brian, May 2007

The main difference between hot water and c/heating is size of the flame as the c/h can be set lower than the maximum for smaller properties. It modulates for hot water as well but only when the water is quite hot.
It may be that the low flame setting on boiler isn't quite right and the flame rectification system isn't picking up the signal.
Your engineer should be able to test for this.

T.H.S, May 2007