Thought I'd follow this up with the final solution!

After poking around with a multimeter for a while and not finding
anything obvious with the boiler stat or air pressure switch,
eventually bit the bullet and got a local boiler guy in to take a
look. He poked around with a multimeter for 10 minutes or so and
diagnosed the problem to be the PCB. Was initially suspitious as this
happens to be one of the more expensive components! (£90 or so), but
was at least quick to fit. Eventualy went for a reconditioned PCB at
£30 or so. Seems to have fixed the problem. £130 incl. seems small
price to pay for working CH at this time of year!

Many thanks for all the help- was worth a try doing it myself!

Cheers,
Dave.