This Norman archway is the oldest part of the church, dating from the 12th Century. I understand it wasn't originally here, but the stonework was salvaged and reused when E.G. Paley rebuilt the church (apart from the tower) in 1865. The tympanum (recessed inner section of the arch) depicts the temptation of Adam & Eve by the Serpent.
The repurposed archway wasn't intended to be used as a door, so, strangely, the builder blocked it off with mediæval decorated coffin lids.