See all the mechanics that I've seen fill a Vauxhall diff with oil is via this plug as when it overflows it means it's got the correct 1.8/1.9 litres of oil. Apparently the selector housing hole it just a breather. either way works so don't really bother me.
May be something I will just make when everything is sorted as it's not urgent.
I had a 32mm socket but that only covered the inner metal section of the seal and bashing it bent that section. May have to find a metal pipe like you said Benn.