Is my clutch the problem?

Recently when I accelerate through the gears it over revs making acceration slow and you have to keep backing off the accelerator.

The worst is in 5th though which seems to constantly slip and only accerates when you very lightly increase throttle. Is this the clutch or the gearbox? I don't know a whole lot about engines so lamens terms would be appreciated.

I have a freelander 2.0 with nearly 100k miles on the clock and no service history so I can't check to see when anything was replaced.

Manual or automatic box?
If manual, then clutch.

Does the pedal feel 'normal' and springy when pressed or stiff and vague all the way down and up? Try to move off in 4th or 5th gear. If it comes about all the way up without stalling then it's clutch slip. You can get a clutch for less than £100 - plus fitting of course.

The car stalls when I try to lift the clutch in 4th and the clutch pedal feels normal I think. It's always been a little stiff.

Still sounds like the clutch. Try actually taking off in high gear rather than just lifting the clutch out slowly. If it mimics what you get on the road and tries to rev then its the clutch plates more than likely. At 100,000 miles it's no shame to an original clutch. You'll notice a lighter pedal too once its done.