Just a quick update, its now almost 6 weeks since I dealt with the problem and I can report success! I hate finding threads about problems and never hearing how it was resolved.
After gaining access to the gearbox linkage / cable ends I removed the cables to test and found they were free and easy to move, since I was there however I took the oppurtunity and put oil down the cables.
Removal of the boot covering the linkage allowed full inspection of the linkage and its various moving components which had originally been lubricated with some for of grease. This grease I found had gone hard and was no longer acting as a lubricant, the moving surface were also now displaying corrosion leading to resistance. After some akward manouvers I managed to remove the old grease and clean up all surfaces which made a noticable improvement to the operation of the linkage. After liberally applying oil the gearchange was excellent, after a weeks use and continual improvement it now operates like new. After a weeks use I throughly greased the linkage and replaced the boot.
Now 6 weeks later its still like new.
Thanks for all the help earlier in the process.