here is the problem. The car was not used for 8 weeks. It was started during this time but was hardly moved. It ran ok initially for the first time, now the it won't change gear, but only when running, when the engine is off and the clutch pressed in, the changes through the gears are smooth as before. But when the engine is running the it will not move through the gears.

 

Your master or slave cylinder is toast. Best to replace both.