I was out detecting this evening for an hour after work and found this amazing bit of history.
It is a lead seal that originated on a document from Pope Martin V at some time between 1417 and 1431. These official documents were called Papal Bulls and were dispatched to all the parishes in the Medieval empire.
This example has later been made into a spindle whorl which is why it has a hole through the centre. Lead whorls were used before spinning wheels were invented to make yarn of sheep's wool.
The inscription "SPASPE" stands for "Sanctus Paulus Sanctus Petrus" and the two busts are of Paul and Peter.
The reverse has "Martinus PP. V" where PP stands for Papa. It is astonishing to hold such an important object that was lost just a few hundred yards from where I am now and hasn't been seen for 600 years.