The saint is shown riding bareback in the nude, an unusual state for a Roman soldier. The motto, Decus et Tutamen, started life as an inscription round coins to prevent clipping, the practice of removing just a bit of the precious metal in the hope that the person you passed it to wouldn't notice. It means 'an ornament and a safeguard' and it appears on many new pound coins even though they have no precious metal to make clipping worth the effort.
Sadly, even Pevsner was unable to discover who carved this little gem.