Partially stripped the gearbox down to reveal the selector rods, the reverse/5th gear rod was scored causing it to ocasionally bind in the bore in the gearbox housing at the clutch end.
The selector inhibitor pins would then prevent selection of any gear, eventually the binding rod would free up and gears could be selected normally.
Removed the score marks from the shaft and bore and re-assembled the gearbox.
All now ok.
I have attached an image showing the scoring on the selector shaft (furthest right of the three selector shafts)