The markings are very faint Pete, couldn't photograph it. But I've definitely identified it via Google images. Wrong George, it's a George III copper halfpenny (which were minted 1769-75). A couple of years too late to have been dropped by Stukeley.

Ebay? Nah, I'd better submit it to the PAS....

"Can you indicate the find spot sir?"
"Yes, it was next to a big hill thingy that EH sent someone to dig a big trench all round. They just left it there on the surface".
"Mmmm, sounds promising, we'll go and look."

pete ? now this coin business is rather wierd..i was in the area on the day of the storm.the same storm u took the picture of the rainbows,on my way back to the van i had to take cover and i found a coin to !!!!!! this is a george 2nd coin with britannia written on it.its copper but the date is obscured by patina.i wont elaborate on where i found it but if you drop me a private email im quite happy to let u know plus donate it to the museum if theyre intrested. [email protected]