Is the input shaft on an IRS gearbox supposed to have no play up and down/left or right? Or is it quite normal for it to move a couple of millimetres? (Im calling the shaft that slides into the clutch and fly wheel the input shaft)

quite normal for some play there

Its normal, the input shaft is supported by the spigot bearing in the gland nut on the flywheel, so make sure its in good order before putting the engine back in.

yeah cheers guys i think the spigot bearing is fine. I'm going to replace the seals and gearbox mount while its out.