Well just had full service would they have checked gearbox oil? Afraid I don't know how to do that myself

Feels like it could be that, which is worrying coz i'd be leaking g/b oil then? Prefer that still, coz the clutch/cable thing is always a bit hit and miss. Garages never seem to be able to say what it is. They just like to replace it all starting with the cheapest until it's fixed - guess that's all you can do.