Could the word you can't remember be 'thermocouple'?

The Potterton Kingfisher is a conventional boiler and therefore fairly simple in operation.

You don't mention if the pilot light is 'on'. A blue-ish flame should be visible from the small window at the front of the boiler.

If the pilot light is on, and your Corgi man has replaced both thermostat and thermocouple, it may be that a) the gas control valve has failed or b) one of the fail-safe thermal cut-outs has tripped or failed in 'safe' state.

John