Came across this site: www.mytyres.com
If you follow THIS link, you can find one of their recommended tyre fitters, nearest to you (on right-hand menu, under "Your fitting service companies). One about 4 miles down the road from me, and have to say, I like the pricing!
Price includes dismount of old tyre, putting in new valve, balancing, mounting of new tyre, and disposal of old tyre. All for £6!
Only a couple of quid cheaper than elsewhere, but look after the pennies...