The essence of the film is the understated background presence of the enigmatic and real-life Baroque London architect Nicholas Hawksmoor. The central character (Puritan, of course) mentions that Hawksmoor is the supposed architect of his inherited home and, for no given reason, he arranges his meetings with the mysterious Mr. Grey in the interior of a Hawksmoor church. Nothing else is made of the Hawksmoor link, but it doesn't need to be - in fact, as I can testify from experience (1994 - St Anne, Limehouse, I was with two colleagues), Hawksmoor surroundings can produce experiences redolent of the fourth dimension, and a crucial scene in the film points that up as the central theme.
This is all relevant, as the whole film takes place in an Alternate London (a fact made apparent, for example, in that the film's version of Aleister Crowley has a different name spelling [Crowleigh] and is the exact opposite physical type to our own Crowley.)
None of this is essential to then see the film - which creates its own Victorian tinged world, shot in evocative reds and blacks. Nick Moran is at his best as the faux-medium title character Simon Puritan (with an oddly olde-worlde cockney accent just right for this film), Georgina Rylance gives an extraordinary turn as the noirish heroine (as the Director points out in the commentary, she appears different in every scene) and David Soul and Ralph Brown give solid support. A delicious mixture of the Supernatural and Noir, gothic interiors and quirky characters.
A particularly enjoyable humorous scene sees two Victorian/Contemporary policemen interrogating Puritan.
This is one of those that demand rewatching as there are always new details, new clues, to pick up.
The only criticism I would level is the occasional bad language that feels like it has been tacked on by lazy necessity - I thought it slightly damaged the spell that the film casts.