Muppett - the pub is about 3 or 4 miles south of Selby, on the road to Snaith. It is about the first thing you reach as you approach Camblesforth from that direction. I don't have a clue if there is any public transport going that direction, but if you are arriving at a sensible, known time, it might be possible for somebody to collect you from a railway station.
It also seems to possible to get a train to Snaith, but they are few and far between.
The address and phone number for the pub is as follows. If in difficulty you will be able to get in touch with someone during the gathering.

Black Dog Inn
Selby Rd Camblesforth
Selby
North Yorkshire
YO8 8HX
Tel: 01757 618247
BLICKY to a map

Quack!
Geoff the Duck.