Steve220 was saying that you keep the complete, assembled pot, including rubber boot, in the diff. There is really negligible risk of dropping balls or getting dirt into the joint. It is the correct way, clean and hassle-free. See the Rover manual.
It would be very bad practice to try assembling with the inner parts of the pot on the driveshaft and just the outer body in the diff. They "should" not be removed that way either, as it usually ends up with dropped balls, and not getting things back in the same orientation, resulting in unnecessary wear. Again, the correct way is to pull the shafts out of the inner members of the pots, which has the advantage that the oil stays in as you lift the engine. But you would almost certainly need a special tool 18G 1243, and just try finding one these days... (One was sold on Ebay on 5th July 2015, sadly I missed it.) The aftermarket suppliers have sadly not seen the need to remanufacture it (which would be fairly simple), doubtless preferring to sell more pot joints after ball-dropping incidents.