I think the original poster is talking about a G-reg.
The bi-annual pitot check referred to above is done on N-reg only. This can cost up to £500 depending on how far one is away from the nearest firm that has the special equipment for doing it. A lot of people use a firm in Cranfield which charges best part of £200 for driving down. Mind you, flying to Cranfield isn't that much cheaper nowadays
On a G-reg this check isn't done - presumably because the UK doesn't have any significantly elevated airfields so nobody really cares about altimeter errors.