Hello Guy,
This evening I checked the detent plunger on the right hand side of the engine, this was fine and working well.
Next took off the clutch case and removed the selector ratchet mechanism and plates etc all which seems ok but with a little age related slack mostly caused by some wear in the selector lever shaft bush in the clutch outer case.
I took your advice and connected some mole grips onto the selector drum shaft splines and went up and down the gears while gently rocking the clutch or rear wheel: the shaft was quite stiff and difficult to move to select the lower gears 1st, 2nd and 3rd, whereas 4th 5th and 6th were definitely easier with shifting back down the range being more precise.
Interesting to note that the difficulties in movement with the selector mechanism removed were as replicated when they were connected and worked through the shift lever.
This has more or less confirmed my suspicion that the problem is within the gearbox somewhere but unfortunately it is not a place I am familiar with.
Where to go from here? The bike is great and having apparently not being started or ridden for years it now starts, runs and pulls really well but I am a bit reluctant to open the gearbox up. But I like things to be right and ultimately it may be the only way to see what is causing the problem and get the bike right again.
Thanks for your advice anyway, it's much appreciated. Joe