Tyreshopper use National, (My mate is the manager in the shop i use so does a good job on fitting them and can swing me a deal as well over tyreshopper) Not by much but better in my pocket.
Camskill are no longer as cheap as they used to be. Not sure if was a deal or not but a friend of mine got a better price from Kwik-Fit for 2 front tyres for his S4. Similar size and Bridgestones too.
Saxongate Motorist Centre at Biggleswade - excellent prices. Added to which, they are super careful with your alloys. Good blat down the A1 for you Simon