You called melud'
Almost right Hortimech,
If you book a site remotely in advance (Post, phone internet etc)then Consumer Protection (Distance Selling) Regulations 2000, apply.
See
http://www.out-law.com/page-430
"The Regulations give consumers a right to:
receive clear information about the supplier, the goods or services and the sale before deciding to buy;
confirmation of this information in writing;
a cancellation period of 7 working days in which to withdraw from the contract;"
In this instance I believe the restrictions on the use of BBQ's are a significant term, and give good cause to cancel unless it was discussed during the initial negotiation. But to cancel under the distance selling regs, you must cancel within 7 days of the initial agreement.
I suspect that the OP has left it too long to be granted a cancellation under the CPDSR2K's, but there is no harm in raising the issue with the site operator pointing out that customers should be told of the sites restrictions before they part with money.
Hortimech wrote
"If you were not given a chance to read and understand their terms and conditions before booking, I do not think they can enforce them on you"
I'm not sure Hortimech wanted to imply that you could use the site and ignore the restrictions, That is incorrect. You were made aware of the restrictions when you received the written confirmation. You had the opportunity to challenge them, or cancel you booking within the 7 day period. The fact that you didn't cancel or challenge means you have accepted and agreed to the full T&C's.
Your rights may be different if the restrictions were not included in your booking confirmation, and you only found out about the restriction when you arrived on site. It might be tricky to prove that the loss of that facility was of sufficient magnitude to warrant refusing the pitch and claiming a full refund.
The moral of this story is if a particular facility or service is of significant importance to you make sure you ask or clarify the details before or as sonnas possible after you pay.