A base silver argentaeus of Licinius I AD 308-324.
Reverse is Jupiter on eagle. IOVI CONSERVATORI AVG. Index mark Trier (PTR). Seaby 3792A.
It is base silver, weighs 3 grams and is 16 mm in diameter.
This is a rare find for the UK.
I hadn't come across this denomination before, Wikipedia tells me: The argenteus was a silver coin produced by the Roman Empire from the time of Diocletian's coinage reform in AD 294 to ca. AD 310. It was of similar weight and fineness as the denarius of the time of Nero. The coin was produced at a theoretical weight of 1/96th of a Roman pound (about 3 grams).