[1] The architectural historian Nikolaus Pevsner describes it as being "an astonishing church",[2] and the citation in the National Heritage List for England states it is an "exceptional form of Catholic chapel for the period before Emancipation".

Its plan consists of a five-bay nave with a clerestory, north and south aisles, and a polygonal apse.

[1] On the underside of the middle corbel is a bust of John Butler in military uniform.

[1] The ceiling is rib vaulted with carved bosses and there are four-bay arcades with dogtooth decoration.

On each side of the altar is a carved relief, one showing the Beheading of St John, the other Mary Magdalene.