Many larger towns have a Calor gas depot (assuming you're using Calor of course). You're guaranteed to get best prices there and advice too. We paid £13.25 for a 4.5kg refill.
I think on site you'll pay extra for the convenience.
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