Apologies for the lack of feedback but understandably with all that's going on I haven't had a chance to get back into the workshop. Nevertheless, I did have a go at the gearbox this weekend. Stripped it and had a very careful look at everything. The innards are in really good shape and I would be lying if I said I found something really obviously wrong. The only thing I could see was that the spring pushing out the locating pin which locks the camplate seemed really stiff which made rotating the camplate quite difficult - even without the innards. I found no literature on if the spring has a certain length or stiffness or if you need to add shims between the cap and the gearbox to alter the stiffness, so I just put it back the way it came out. The other thing was the timing of the camplate and the selector arm (where the ball fits). Some say you need to time it that in 4th Gear the arm lines up with the top right gearbox stud, and others say that at the extremes, the distance from the notch from 1st and 4th need to be the same. I set it up this way, but I haven't finally reassembled the box, so I can change it if there is a more scientific way to get the timing right!