Best bet is to look on the internet, find a tyre that you want, then call local tyre outfits and play them off againist each other, obviously you'll get a big discount if you buy 4 tyres, and pretty small is you buy 1. But I managed to get a set of 17" Pirelli P Zero Neros for £380, with a £50 Sainsburys voucher thrown in from a local ATS Euromaster. They even came to my house to fit them. Black Circles and the like couldn't touch them on price. At the time the Neros were £100 - £110 a corner on internet fitted sites!
Always worth shopping around, especially on popular tyre sizes. Best deals on
205/55 R16
215/45 R17