Secondly, if an airport runway has multiple instrument approaches, ie. ILS30, VOR30, NDB30, in the UK how do you know which approach is in use?
All of them - the one's you quote are for the same runway. You can either pick the type of approach you want (if you're training) or you'll get told which you'll have . If the Controller has a sequence going and wants the least delay for everyone to get every aircraft down then expect the latter to happen - but you may get offered a choice.
However - if you pick a full NDB procedure when everyone else is being radar vectored and it's busy then expect to hold at a higher level and have a long wait
CM