I've always found Blackcircles very good for price - have a look for prices - http://www.blackcircles.com/ . Plenty of choice of makes with user-comments on them.
You can choose a local garage (or mobile tyre fitter) and a time to suit you.
Do you have an f1 autocentre near you? I rate them highly (they are still family-owned), they have Avon Tyres and will beat any online price. See what they are charging for the same tyres.
Baz......try tyreshopper...this is National tyres own website.You can also download a discount voucher code to reduce the cost. NTS quoted the wife £702 for a pair of rear tyres and after doing the online deal had them fitted at NTS for £560