If anyone has done this job themselves could they please tell me how to do it in easy stages. Is it a matter of just undoing the large nut, removing the old unit and sticking on the new one, and also are there any pitfalls to look out for like that bit falling into the gearbox the Haynes manual warns you about!

I'll take that as a "no" then! Yesterday once I got going all the instruments including speedo and odometer worked only to not work on the return trip, so I don't think the sensor is at fault if it can work some of the time. After exhausting all other avenues like earthing points, fuses, dashboard circuit etc., it seems it's something I'll have to live with - annoying, but not worth the hassle of trying to cure it!