You can see the date at the bottom of the coin, 1735. Above it looks like three letters,
This side is far more difficult to decipher. We think the bit to the left of the corrosion at the top might be a bird head and neck, with a wing on the right side of it. But it might not be, it might be a scroll, it might be anything. This picture is actually pretty good, you aren't missing anything in the photo that we can see in person.
So if anyone out there has ideas of what this coin might be, or where to research it more, please leave a comment!
EDIT: Through the power of the internet and my supergoogling friend, Babs, we have found that the coin is a Russian Denga. Not worth a fortune, but an interesting find. What is even more interesting is to ponder how it ended up in an allotment plot in Cookham???