SUCCESS!! ...or at least it seems...
Switch arrived from Emmotts today. Got the old one out in-situ... It has 36mm flats on it, but not enough clearance between the switch and pump bodies to get a big socket on it. I just about managed to get it initially loosened with a Viscous Fan spanner, but then had to undo the pump mounts to move it around and get enough of a turn on it to remove it completely. Be aware that the switch body is milled Aluminium and is easily damaged by a steel spanner! Had shedloads of brake fluid come out, so a small thin tray of some sort underneath it would have been a good idea... The absorbent cloth I stuffed under there was quickly overwhelmed! Thankfully, the brake fluid followed a path that took it away from the top of the air spring, hoses, anything rubber, etc so hopefully I got away with it... Just had to thoroughly clean it off the bodywork.
Now, pump runs as soon as the ign is turned on, pressurises the system and stops after a seemingly sensible time. Brake pedal is nice and firm too. Won't get a chance to road test it until the Nagger get back from work... Not too keen on having the Clarkylets in the car until I've proven the brakes work!
Thanks for your info and research on this matter. I have just had mine diagnosed with faulty pressure switch.
Is this something I'll be able to do with limited Mechanical experience? I will be phoning Emmotts in the morning to try and order the pressure switch for the weekend.
Are there any special tools you'll think i'll need.