I've just spent 20 mins outside with a variety of tools and half a can of wd40.
It's fixed!
Reading into this a bit more, there's a small pressed steel arm on a pivot that the shift arm uses to control the in/out movement from the tower (this equates to the left/right action of the stick).
I'm guessing the minor coolant leak I had in the area had affected the free movement of the pivot.
Removed a clip from the end of the pivot and eventually worked it out from the bush. This reluctant movement was in itself was a bad sign. It was all pretty manky looking and the pivot slightly pitted.
Checked the motion of the shift tower and it was free moving and returning to centre again. Good sign the tower springs are OK
Polished the pivot, flushed out the bush with wd40 and all refitted.
Works a treat again. Happy days