I had a few hours spare this afternoon so fit the new linkage and mount.
While relatively straightforward and easy to do, getting the two mm bolts out of the plastic housing was a sod of a job. Anyway some scuffed knuckles and a few swear works later I had the housing off.
The mount was as I'd thought completely shot, the rubber was perished and the plastic insert was only just held in place allowing for a lot of movement.
I removed the 13mm nut from the end of the linkage and after prying the sleeve open a bit it slid straight off.
The plastic universal joint was seized solid and I was unable to move it by hand, that coupled with the knackered mount explained the stiff yet vague gear change.
I slid the rubber boot off the old and back onto the new then fit the linkage. Slid the new mount onto the bottom half of the housing and re fit.
The difference is night and day and feels so much better, smooth and precise.