3a Seymour Walk: St Dunstan's Priory.

Pete Shotton picks a party chez Lionel Bart, held on 9 October 1965 and attended by all four Fabs, as the epitome o' Shwinging Shixties shiznizzle. Bart, extremely successful songwriter that he was, owned "the Priory" (pictured above) in Seymour Walk from 1964-1968, and here it was that the aforementioned wing-ding shwinged. (Get togethers round Lionel's were a regular occurrence, and featured all the accoutrements one might expect. For related reasons, Brian Epstein was both good friends with Mr Bart and a frequent attendee.)
Here Lionel is in the first floor drawing room:

In Pete's book, a couple of Polaroids show Cyn, Patti and Beth Shotton reclining on the remnants of some poor beastie in this very locale:

Bart's piano can just about be discerned:

A couple more "areas":

John apparently loved this place. Quoth Pete: "Much to John's amusement and delight, our gracious host had also contrived to turn his house into a veritable labyrinth of trapdoors, hidden stairways and secret passages. Wandering about the house, John and I were constantly colliding with walls where, but a moment earlier, there had most assuredly been corridors or rooms."
Now. A couple of plans exist, dating from early 1964 and March 1965, and these show the alterations that Bart made to the Priory - here is a bearings-getting red arrow:

Funnily enuff, said plans show no evidence of jiggery, or indeed, pokery of the type that Pete mentions. A couple of doors get altered, the odd window is fiddled aboot with, but that's about it:

However, the ground floor was a rabbit warren (not literally - that would have been ridiculous), and this fact, combined with the freely available substances that were a mainstay of such festivities, might account for etc.:

Lionel 'n' John, John 'n' Lionel:

And that's yer lot.