My Potterton Profile 60e LPG will not fire up?

When the heating or hot water triggers, the fan starts, the lighter clicks and the pilot lights up, but the main boiler does not fire.

The fan seems to work... I've tested the flu and it has good air flow and I've had the boiler open and the fan spins quickly and smoothly.

The pilot lights, so it's not a problem with the starter.

Could it be the air flow switch? If so, where is the air flow and how would I replace it?

Help gratefully received.