Tim,
When you say you want existing residential land to be redeveloped do you mean that large amounts of existing housing across Forest Hill to be demolished and replaced with modern three-storey flats and town houses? Or are you suggesting that home owners should be encouraged to extend their mortgages and convert their attics into penthouses for young families?
By targeting existing residential you are either expecting existing home owners to buy into this scheme to split up their homes, or you are expecting streets to be bought up by developers for systematic redevelopment.
Or is there another solution that I don't see?

I still prefer the option of developing brownfield sites (and some green field) around London which are not in use. I see no point in keeping playing fields (in Anerley) in public ownership when they have not been used for 20 years and a single site could provide more housing then large scale redevelopment of streets in Forest Hill. I'm not against converting houses into two flats to some extent, but I don't believe this can be the primary solution to the housing crisis which has resulted in such high rents and property prices.