Tom, go have a look at:
http://www.kites.org/jo/stores-uk.htm
My son and I fly kites - he runs the big ones used for buggying and I have two stunt kites. Prices range from a few quid to £500 plus. Please be advised that the larger ones can be highly dangerous in inexperinced hands. In mild conditions at Kew recently my son watched somebody launch a big kite for the first time - ten minutes later he was in an ambulance with several broken bones having been hoisted upwards and dumped unceremoniously!