Had another look today, and found it, but very, very difficult to access. Decided to cheat and run it round to the garage that we have at work. Apparently, the fitters there had done many of these before and were debating whether to do the job with or without the blindfold!
They by-passed the switch, just to check that was the problem, and just one hour later I was bringing the van home. It seems the hole for the reversing switch is also the filler point for the gearbox oil, which is very surprising considering how inaccessible it is.
Anyway, thanks for all your imput, really appreciated.
All the best, Rog.