I recently had to replace a damaged fornt tyre which was virtually brand new.
To match the other side with the same tyre, I started an internet trawl for a good price on an N2 rated Conti Sport Contact 2 225/40ZR18. Having tried the usual suspects recommended on Porsche forums, I rang a local dealer, Edgwick Tyres (01295) 271362, and they gave a price of £123 inclusive of fitting, balancing etc. This price was within pennies of the cheapest 'supply only' price I could find on the internet, so if you are in the Banbury area, they are well worth a call.
I think they also have a place in Chaventry or Covenrty as it is more widely known.
Steve