Roy
That completely vacant site you mention was know (and still is by some) as The Big Top, that is because after it was bombed during WW2 it was cleared and used for circus performances and when it wasn't being used by the circus it was used as a car park. It served these purposes right up until that section of New Street, High Street and Union Street became the first site to be redeveloped in the mid 1950's.
This set of images show the progression of the big top site.