At the True Lovers Knot at Tarrant Keyneston, Blandford Forum, Dorset. DT11 9JG - 01258 452209

http://www.trueloversknot.co.uk
Yep, we don't even have to walk to the pub, we're camping right outside it!
It's £15 a night including electric (I should check whether it's cheaper without as we don't need it!) and that gives you a discount on meals in the pub.
Need to book up fairly rapidly on the above phone number rather than email, saying that you are with the RiderSite group. They want a 50% deposit which they can take on the phone.
By the way, if anyone wants to wangle an extra day off, there's a bike night there on Thursdays - I haven't been to it yet so have no idea what it's like.
The pub does have rooms which are available Friday night but unfortunately fully booked Saturday night. I'll investigate B&Bs if anyone wants me to.
I haven't done this before so feel free to let me know what I haven't told you, or what I need to organise - I'm sure you will !