Like other posters have said play basses of diffreing price ranges againt cheaper models and try and disdern if the £600 or whatever differnece in price is in the fit & finish, pick ups/hardware or actual playability. Pick ups and that can be changed but if the basic feel and playability are right thats the best measure or a good bass. I replace a lovely jazz bass with some Dimarzio Model J's(which were a steal at £98 new!) and that bass was rockin'!
I currently have two laklands, a 4 and a 5 and the playability of each is what swayed me over the other options available to me at the time. £500 budget for a decent 2nd hand bass, especially off the BC classifieds is the way forward.