I cant select gear when the engine is running, I can select gear when engine is off then start with clutch depressed but can only drive in the gear selected i cannot change gear, is this a clutch problem, transfer box problem or gearbox problem pleae any help would be much appreciated

Hi I have recently purchased a freelander 2000 plate TD4 3dr van, Its hard to select gears, I think/hope it maybe the clutch slave cylinder as if you double the clutch it goes into gear easily. But I have a niggling doubt it is not the correct gear lever as it has no hill descent button on...