Thanks Ian. The disc was in good condition and a very tight fit when I tapped it back in, so hopefully it'll stay there. I'll try your leak check idea, maybe a bit of thinner oil would be closer to warm transmission oil for cold leak checking?
I count myself lucky that the drive shafts came out OK. My biggest problem was hub nuts, but I won in the end!
This core plug is fitted as you guessed to prevent the oil escaping, or having to drain the gearbox if you want to remove a drive shaft. When you tap it back in, even if it seems a tight fit, you should hit it hard in the centre with the ball end of a ball pein hammer. This flattens the curved core plug and makes it lock into the hole so it should not come out again. You don't want this dropping out inside the diff. whilst driving the car!
I have new core plugs in my stock if anyone needs one.
A small amount of swarf on the magnetic drain plug is normal, as long as there are no large pieces in it!
Roy