Just to finish off, thanks for the advice, and sorry for posting to the aircooled group - must get my eyesight fixed, or wake up. For what it's worth, the fault seems now to have gone completely. Looks like the bloke in the VW dealership was right about the water in the sensor. Had been heavy rain, but now drier and must have dried out. Never done this before, but this is the first time car hasn't been garaged.
As for aircooled, best car we ever had was a 1973 1300 Beetle, bought new, 90,000 miles on the same clutch, and sold 13 years later for more than we paid for it when new. Brilliant car - only replacements were tyres and exhaust (stainless one lasted 10 years). Have to say the Golf Mk 4 is also a great car, but when you see the VW reliability ratings these days you have to wonder where German engineering is going wrong. VW's attitude to its customers in the UK is also pretty shabby these days compared with the way they used to be (for example, total failure to sort out a known locks problem on many of their cars), and as a result the next car will almost certainly be Japanese. Shane really.