Jon,
photos of gearbox internals showing gear selector mechanism. I would suspect the gear selector shaft MT569 may be rusted in the gear selector fork MT568. There is a spring and ball in the selector fork which might be rusted too, under the split pin. The only way to access these components is to split the gearbox, if you do this there is a nut and bolt underneath at front that is easy to miss, 9 fixings in total. The two halves will need gently persuading to come apart.
Attachments:

