So it turned out that the metal pivot that changes the horizontal movement of the cable to the up and down movement of the selector arm was seized up and took nearly 2 hours of heating, hammering, twisting, cursing, sweating, and praying, to get it off. Once off you could see where the pin it pivots on was pretty rusted and pitted, and the bushes were very dry. Cleaned up the pivot pin and the pivot itself, and then drilled and tapped a hole to fit a grease nipple. Packed it all with grease and fitted it all back in (which took all of about 5 minutes) and now I have a lovely crisp and smooth neutral movement on the gear stick for the first time in my ownership. It has never been right since I got it and just got worse and worse over time. At least I didn't have to take the gearbox out! Anyway, thanks for your help people.