Glow-worm 30HXI F5 fault code - cannot restart boiler?

When I switch off, turn the RHS switch to 0 and the LHS switch to off and pause as instructed, it usually starts again when I reverse the process. This has happened a couple of times in the last month.

Tonight it tries to start for a couple of seconds then fails. After a pause it makes a more convincing fist of it, but after about 15 seconds it fails completely.

I have taken off the front and the inner silver cover, but I cannot see anything else to press. I have read there is supposed to be a thermostadt reset switch in the top left corner of the box in diagram 10.2 of the servicing manual, but I cannot see anything useful to do with it.

Anyone got any bright ideas?