Thanks for the latest suggestions. I think I need to turn my attention to the spigot bush as it's pretty unlikely the local garage put the clutch plate in the wrong way round... They are extremely good, know their stuff inside out and the bloke who did mine suggested the clutch kit and is a Discovery enthusiast. I did half-jokingly suggest it to them and they looked quite insulted! I've got the flywheel and clutch components sitting in the boot, they look like they just came out of the box.
Would an eggy crank be noticeable or would I need to measure with calipers?
When I'm not thinking about Discovery gearboxes I'm trying to run the farm I rent. Currently coaxing an elderly combine through the crops and dodging the showers so haven't got on with this gearbox problem for a day or two. I'd better send the box back to the rebuilder, though he's not keen on standing by his guarantee as he swears blind it's not the box... and this Discovery does have a history of eating gearbox input shafts, to be fair to him!
Thanks all, your help is greatly appreciated...