In reply to AJM:
I think you need to factor in the difference between roped centres and bouldering only centres though, I can only speak for my situation (my wife isn't a big bouldering fan) but quite often we will specifically want good roped routes which requires casting the net a bit wider.
Personally I think Southampton is a better location than Salisbury, if there was a good, 10m+ roped centre in Southampton, the only competition within an hours drive would be Calshot, but due to the road network, the vast majority of people would have to drive right past a wall in Southampton and then drive a further 30 minutes to get to Calshot, so why would you bother?
As harsh as it sounds, a good Southampton centre could steal a lot of Calshot's custom, along with the catchment for 45 minutes in all other directions, whilst keeping on good terms with the two big boulder centres either side (Red Spider and Project) and letting Golden Gecko keep the kids/introduction climbers.
I'd have opened one myself if it wasn't for the several hundred thousand pounds that I don't have to start it up...