Hello, I hope the following assists you.
From the description you've given this does appear to be a case of the selector mechanism seizing up in the turret.
To successfully grease the turret requires it to be removed and the shaft to be taken out of it, usually the bearing needles are too corroded to be able to make a robust repair and replacing the turret is the best option. If the bearing needles are OK then the shaft need cleaning and the bearings cleaning before refitting the shaft with grease in the bearing. Make sure the transmission breather pipe is OK and located correctly as otherwise water gets into the turret causing the problem again.
The hotter the engine bay becomes the tighter the shaft will be to move.
If you haven't removed the shaft then the chances are the problem will still be evident or will reoccur quite quickly.
I hope this helps but if you have more questions let me know and I'll reply as soon as I'm able to.
Thank you for your answer I have emailed it to my mechanic.
He has taken the turret apart and not sure if he has checked the shaft.
He has greased it and cleaned it all and it appears to be running fine.
I haven't had it on a long run so if it stiffens again we will have to look further into your answer.