If Lewisham aren't acting then I would think about a different way of letting them know.

They've got a good website based reporting system for graffiti, flyposting, bins, etc etc. They even have an iphone app I think to help. Also you can send them a picture message of the problem.

I've reported several bits of flytipping around the place and this seems to get it cleared quickly.

www.lovelewisham.org

If that doesn't fix it - try your local ward councillor.