I had a quick look at the Environment Agency document on flooding in S London.

I've not come across local flood authorities before, despite service on two councils with some areas of high flood risk, so I referred myself to Mr Cyber' electrickal source of all knowledge, and have thereby identified that the Local Flood Authorities are the Counties and the Unitary Authorities. I would assume that, in London, this must be The Mayor of London (ie Borisius the First), rather than Lewisham which is a second tier authority.

Would suggest that Ewelme Road residents, and also the Lewisham Councillors contact the London Mayor's office, and also speak to Len Duvall who is the Assembly Member.

As an aside, if the above is correct, it's a bit surprising that Lewisham officers, Thames Water staff, and the Environment Agency have not pointed people in this direction.