Ok, update...
With it all in neutral I can hold the syncrohubs and wobble them about 2mm each way. This is whats causing the locking up. The syncros are wobbling accross and the baluk rings and gripping on the tapers of the gears.
That's not good, I need new syncro hubs and main shaft.
Changing the selector forks will stop the wobble. And they are much much cheaper. It would probably be ok when its being driven properly.
That sounds sensible, the original baulks would have worn as the forks did.
