Dude where's my clutch

I've just swopped engines from a 2.5 na, to a disco 200 tdi, on to an lt 77 gearbox a few niggles but got to the exciting part when the engine finally ran, thought I'd take it for a quick drive to check it out, pressed the clutch, but couldn't select a gear, pedal felt normal, and if you forced the lever towards a gear, the vehicle crept forward a little, or ground angrily with reverse.
It drove normally prior to removal,I'm confused convinced id put the clutch in wrong or forgotten the release bearing, I removed the gearbox good chance to fit the new transfer box. Clutch plate is in the correct way round it won't actually go the wrong way, and release bearing was in place, clutch fork is ok, and had previously been reinforced, checked operation of clutch slave which appeared to operate fine if I stuck a screw driver in the end and pressed the clutch pedal, checked the clutch plate wasn't worn or stuck, it's quite a fresh one so I re-used it from the old na engine, I can't work out what's wrong, is it because the clutch friction plate is wrong or ave I missed something, I really don't want to take the box out again if I don't figure it out now. Any ideas

Have you tried to bleed it up?
I would guess some air has got past the slave seal as its gone to full stroke position with the engine out.