Having read threads elsewhere I tracked down the cause of the gearshift stiffness to the bushes in the selector plate on top of the gear box. Got some new bushes today and also some silicone grease (supposed to be the stuff to use with plastic bushes?).
Most threads I've read suggest removing the battery tray. However, I managed to change these bushes with the tray in place, though with the battery removed, I guess it depends on the size of your hands to squeeze between stuff. I also read many folk using a hammer or mallet to remove the selector crank bolt, but I found it easiest to use a screw driver behind the crank to lever it out from behind and thus remove the potential for damaging threads.
Removing the old bushes was a bit fiddly but the rear one came out easy enough with a screwdriver pushing it out from inside the shaft hole, the front one was more stubborn but gave way in the end. As recommended I used wet and dry paper to clean out the shaft bore before smearing it and the new bushes with silicone grease, once re-assembled all was self-centring and nice and smooth
Took me about 45 minutes in all.
However, having fixed the shift problem, the clutch is getting worse and I reckon it is the clutch actuator bushes and/or an issue with the slave cylinder. As a short term fix I sprayed the top of the actuator arm bush with silicone lubricant, I got my daughter to work the clutch whilst spraying it, but was surprised by how little movement of the arm was achieved, this leads me to suspect that the stiff actuator may have caused the clutch hydraulics to develop a problem, at best I suspect it needs bleeding, but perhaps a new slave might be order of the day as well as replacing the actuator bushes. I've got the GT booked in for a full service, cambelt and water pump change, might ask them to have a look at the clutch issue too, reckon this is going to be a big bill, but it should be all the key residual issues with this GT sorted, leaving me to tinker with iPod docks, bluetooth and also repairing the driver's seat bolster.