The lease prohibits the display of For Sale sign. So the lack of signs does not mean there aren't any flats for sale. It's a very good idea as it prevents a permanent plantation of For Sale signs.
Turnover of apartments is very slow. I think I am holding the current record at 35 years but many have been in the same ownership for over 20 years. The majority are occupied by the owners.
A rummage online will obviously display flats which are for sale. A suggestion by me, one agent has specialised in the estate ever since his parents lived in the flat below mine.