Just in case anyone was interested, the fault was caused by a defective PCB - or far more specifically, a defective relay.
The relay activating the fan had suffered heavy arcing of the contacts and no longer made a reliable connection.
It dawned on me that it couldn't be an air pressure switch fault if the fan hadn't had a chance to "spool up" to MAKE a pressure difference to detect in the first place. Applied 240v directly to the fan connector from the PCB and fan spun straight up and allowed the boiler to fire.
Then tracked it back to the relay which, once replaced made it all happy again.