Thanks for all the replies; I shall give it a try (except for the gas valve) and report back if it does happen again.
What puzzles me actually; it happened the first time the boiler was installed. (on the first run)
The engineer knocked the bottom of the casing, and it fired up.
"That will be...
This is a condensing boiler about 1 year old.
It is working ok, but about once a month the orange warning light on the control panel warns of flame failure of the main burner. I used to give the casing a knock, and it would fire up. Manually turning the boiler thermostat to on/off does fire it...