Dean,
As far as I recall, the first thing going 'into' the gearbox behind the selector lever is a small o ring seal on the selector shaft, so that keeps muck out of gearbox. It is separate from the problem area around the detente ball.
If you have the PRM manual with the drawings in it, the bits we are talking about are-
C4 operating lever
C5 steel ball 6mm
C1 spacer
C12 'o' ring
Etc.
The grating sound when operating the lever in my case was from the area around the spring actuated detente steel ball.
Whilst we are talking about that particular seal, C12. On another occasion after we had put a "few" hours on the gearbox some years later we started to get a small but persistent oil leak out of there. Didn't understand the arrangement at that time so just kept cleaning it up. Then, one day the oil ring failed (split) and it leaked profusely. Expecting a big bill, I called in at the boatyard at the top of Stockton locks to get it sorted. Twenty minutes later and just £10 lighter it was fixed with a new seal and was leak free once again. Great service.
Good luck,
Bob.