Steed's flat in Series 1 - or the curious case of the missing flat.
The scripts mention a flat for series one, but no photos of its interior survived.

"Later after dinner (with Steed), Denise is lounging on the sofa in Steed's flat."(Ashes of Roses 92)

"In the kitchen of Steed's flat, later that night, Keel is washing his hands."(Ashes of Roses, 99), while "Carol and Steed are sitting on the sofa" (91) in the living room (Ashes of Roses, 99).
Unfortunately, no photos from that set exist.
There's a kitchen mentioned in Ashes of Roses (99). And Steed has his first dog in the flat. "Keel looks to Puppy" (100) and Keel "exists with the dog."
John Steed lived in four different flats during The Avengers.
It looks like there's a countdown going on: 5, 4, 3...

Flat 1:  5 Westminster Mews, S.W.1

Flat 2:  5 Westminster Mews

Flat 3:  4 Queen Anne's Court

Flat 4:  3 Stable Mews (City of London or Westminster)

During The New Avengers Steed is living in a mansion outside of London, called
Steed's Stud Farm

The flats at Christmas and when Steed's having a party

Clocks in Steed's flats

"Ici Steed!" - The telephones in his flats

Venus Smith is was never inside his flat.
Steed's Flats