When my AMF gearbox failed (around 290,000 miles) the first indication apart from slightly crunchy change into 3rd for a few weeks previously was leaving the local Tesco and finding that after 2nd there were no further forward gears that I could engage - the gear lever would not go further across the box than the first plane. Selector would also not go further into the box when moving the lever on top of the gearbox by hand either. I had Reverse, 1st and 2nd and that was it. Luckily home only 6 miles away in 2nd. Swapped it with the 150,000 mile GPK gearbox recently out of my BHC (when it was PTW-swapped) and instantly back to full function again.
Realistically it sounds like a proper failure and the easiest replacement would be a MYP/PTU/PTW/NTG 6-speed if you can source one unless you can source a suitable 5-speed maybe when someone else is doing a 6-speed swap (from WOM or within A2oc?). I've sourced PTW / NTG for around £200 each for my two tdis and very happy so far.