RAC. Only had 3 more miles to go till the campsite

it's a 2.0L Aircooled with a CU engine - not sure of the gearbox code

slightly more detailed version:
I first noticed a high pitched whine (like a high speed reverse) in second gear, so I pushed on using third (trying to find a suitable place to pull over), then a couple miles down the road there was a horrible crunching sound for a moment the a knocking noise, changed into 4th and all was as normal for another mile then the same horrible noise, so changed into 2nd, lasted about 500 meters, then the same o
horrible knocking noise, changed into first and limped to the esso forecourt.
any suggestions? cheers, Ben