Just had 4 Pirelli P Zero Nero tyres fitted to my car (205/45/16) at the ATS Euromaster in Godalming.
Price came to £269 minus £40 Sainsburys voucher. Which works out at just over £57 a tyre taking the voucher into account. The next cheapest I could find for that tyre was around £75 from http://www.blackcircles.com.
The prices vary between branches so phone around and get the best deal then ask your nearest place to match it. A friend found the above price in Slough which Godalming matched.
One thing to note is that you have to send off for the voucher. You're given an application form that you have to fill in and enclose your receipt. Offer ends at the end of this month and voucher has to be redeemed by the end of November.