if you stop the engine can you select gears?
If you can, then the clutch is dragging for some reason.
If this is the case, then
Select 4th or 5th, put the handbrake on. Depress clutch. Start engine. HIgh revs, try to pull away. (Handbrake on)
This will hopefully polish the clutch and flywheel a bit more, allowing it to release properly.
You did put the clutch plate in the right way didn' you?
If you can't select any gears with the engine off, check the gear selector linkage is assembled properly, on the defender box there is a wee collar that is a pig to line up, under the selector box.
Failing that, it sounds like the box is at fault im afraid.