Tried moving it through the gears as suggested by hand. When it changes correctly a definite motion can be felt along with a noise that indicates something happening correctly. However, it can still be felt to malfunction sometimes and when it does it's the same sensation as you would feel if the external linkage was sticking - which i've checked and can see no issues with. At the same time it feels as if the shifter isn't travelling through the same range of motion that it does when working correctly. This occurs whether the engine is on or off and also irrespective of clutch use. The problem seems also to occur randomly as far as which gear it happens with, also irrespective of whether going up or down the box. When it doesn't happen though the change is lovely!

I've topped the oil up no difference.

It's the apparently random occurrence that gets me! Could it be clutch still i wonder?