Steed's and Mrs Gale's flats in series 2:
There are only five appearances (orange) of Steed's flat in series two episodes. The outside of the building, where the location of  Steed's apartment is supposed, can be seen once in episode Warlock.
One can see Mrs. Gale's old flat in Mr. Teddy Bear, Death on the Rocks (renovated), The Big Thinker, Warlock and in Killer Whale the new apartment, that is Cathy's home in series 3.   

Apparently, Cathy does not have a key for his flat, because she rings the doorbell of The White Dwarf. That's very interesting as Steed let her have his flat in The Golden Eggs, which got produced before The White Dwarf.

Mission to Montreal                         
Dead on Course
The Sell-Out
Death Dispatch                                
Propellant 23
Mr Teddy Bear  Mr Teddy Bear                        
The Decapod 
Bullseye
The Removal Men
The Mauritius Penny
Death of a Great Dane
Death on the Rocks
Traitor in Zebra
The Big Thinker
Intercrime
Warlock (the outsite of the building) Warlock
Immortal Clay
Box of Tricks
The Golden Eggs
School for Traitors
The White Dwarf
Man in the Mirror
Conspiracy of Silence
A Chorus of Frogs
Six Hands Across a Table
Killer Whale

BedroomHOMEFlats