I'm assuming you are looking at 215/45? If you dig around the internet you should be able to get replacement Pirelli's for £150 each.
You will find a better range of tyres/prices if you go up a size to 225/45/18 which I'm told fits without a problem but you should inform your insurer of the change from std spec which could incur a charge.
www.mytyres.co.uk
www.blackcircles.com
www.lovetyres.com
Once you get a price you are happy with online it's worth calling around your local depots as they can often match or beat if pushed.