It may be worth you joining Costco if you have one near you as they often do special offers for tyres.
The latest one for Michelin tyres is:
Buy 4 18" or above & save £80 off the fitted price
Buy 4 16" or 17" & save £40 off the fitted price
Buy 15" or below & save £20 off the fitted price
Valid from 21/01/08 to 17/02/08
They are competitive even without the special offers
I am not sure whether you will need the coupon, but these should be available instore.