Hello,
we have quite a few cars and go through a lot at work as well. As a result I always order them online and have our local indy / specialist fit them.
The best to try are:
www.camskill.co.uk
www.oponeo.co.uk
www.lovetyres.com
also this thread is worth a read and mentions those above and more
http://www.pistonheads.com/xforums/topic.asp?h=0&f=23&t=1323052&mid=286670&i=0&nmt=RE%3A+PistonHeads+partners+with+Tyre+Shopper+&mid=286670
our bmw / merc specialist charges £8-£12 a wheel for valve, balance & fitting to give you an idea.
just to add everyone ha favourite tyres I generally use Continental, but have found Hankook ones to be very good with good wear, ride & fuel consumption. I got Falkens fitted to the A5 at work and I am bitterly dissapointed you get what you pay for I suppose.