Went to VW. They put me in touch with a gearbox specialist who, as it happens, had already dealt with the car for the previous owner but through another garage.
It turns out that the garage the original owner took the car to believed there was a problem with the box and sent it to the gearbox specialist to repair. He said the box was in very good conditon, bearings good etc, and sent it back saying to overcome the problem the car needs a clutch and the flywheel skimming, which he would do. But the garage only fitted a new clutch, against his advice, and so the symptoms were only half fixed and the car was traded in by the previous owner and sold to me.
He said the flywheel has probably got a high spot and so creates drag with the friction disc. Because reverse doesn't have syncromesh, that's why you get a crunch.
Solution: Get the flywheel skimmed! But he did say putting the car in second gear before reverse will do the trick... and it does. Top man!
Hope this helps others.
