I don't know about Thirsk in particular, but I've used both Etyres and Tyres on the Drive, who both operate nationally (franchises presumably) and they ahve both been fine
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets