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.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.