Booking is available 3 months in advance.

We offer visitors a facility they can reserve in advance, and during some weekends and peak periods it is fully booked out.

"Fully booked!? Well surely you can fit our group in? There's only 8 of them and I did promise him he could visit for his birthday. He's been showing his friends your website and talking about what he'll do there, he's even asked his dad if he'll bring the camera so he can video their visit..."
"We have some space the following day?"
"Well that's not really good enough. That's not on his birthday."

Well perhaps you should have thought about checking with us first before you promised your son something you can't fulfill.
Oh, and checking more than about four days in advance helps as well.