It is thought that there are only two such stair-cases in the British Isles namely Tamworth Parish Church and All Saints' Pontefract.
There is probably another in Normandy.
The stair-case of All Saints, having a single newel, has the internal
access by a small doorway leading from the North wall of the recently extended Nave, while
the external access is in the corner between the North Transept and the Nave, the latter
being used regularly by the bell-ringers.
My personal opinion is that in the 14th century there was a rood screen,
typically of richly carved wood or stone, separating the nave from the chancel of a church.
The internal access has only recently been restored consequent upon the new extensions constructed during 1966/67.
Formerly it was blocked up by rubble disposed of during the building of the Transepts and fore-shortened Nave and Chancel in 1832/and by the bellows of the Organ erected in 1937. The entrance was plastered over.
Originally, the stair-case led to the top of the octagonal lantern, 82 feet above the ground, as a single spiral (observe the model). But that part above the bell chamber has been replaced by a long stepladder. There were 99 steps; 60 to the old l4th century bell chamber and 39 more to the lantern.
The model, photographed here is in church, you only have to ask to see
References Tew T.W.(1874)Ecclesiological and Naturalist Society, 25th