Wow, that is one super clean mechanism Dave!! My next suggestion would be the tower, although, it has been muted on this thread that it could well be the synchromesh...time to get your spanners out again pal.So I completed the work today. there was loads of debris and sloppiness in the gear-changer mechanism (down from the gear-knob).
Here's a picture of the removed component and its replacement :-
View attachment 69243
Did a test drive .... The change IS more positive, BUT, the problem is still present ! You can make every change in every direction EXCEPT 5th gear down to 4th. It's just like there isn't a slot for it! You get to half-way in the change and you just can't then pull the lever down into the 4th gear position! So what's wrong and how do I put it right? I do have another brand-new tower ..... I've also done the cable adjustment procedure. I'd so like to get this sorted ......