Hi Callum,
I was there May last year.Rock n Roll Heaven on Orange street didn't have much in.Went over to St Petersburg to Banana's but only picked few cheap things up.He has all the price guides plus nothing is priced as he checks on his computer when you go to pay.
There used to be a place on West Church street but believe he sold all his vinyl.Was warned that this area was a bit dodgy,but isn't everywhere these days.
There is a place near University as well but don't don't know name or address.Like Mark said try Dave Moore he should know more.
Dave.