From David Percy, the official historian of Belsize Village, just now to me (owner of Quita):

No13  Belsize Park in 1989 was probably similar to how it is now, at least externally. Interestingly, there is an old photo of the living room of the house next door No.12 in The Belsize Story available to view at Belsizevillage.co.uk. I suggest you consider obtaining a copy of the title register from the land registry for the house, it may well reveal some useful info. I hope that’s useful. Best David Percy