It would be worth suggesting to Southwark that they should ban people with more than four dogs at any time.
Bye law in Wandsworth:
http://www.wandsworth.gov.uk/Home/Enviro...alking.htm

Article from the Standard on Ken & Chelsea
http://www.thisislondon.co.uk/standard/a...article.do

Richmond fine dog walker:
http://www.richmond.gov.uk/press_office/...ouling.htm

Contact Southwark:
environment@southwark.gov.uk
Tel: 020 7525 2000

For the situation in which nevermodern found themselves you should call the police as the owner is not in control of the dog. Tell the police to have the dog put down, that should frighten the owner.

If councils are talking about high pitch noises for youth I would recommend that people walking on One Tree Hill on weekdays take a dog whistle with them. If the professional dog walkers know that the dogs will not go in the area then they will have to take them somewhere else (such as SE15).

Dog whistles to buy
http://www.petplanet.co.uk/shop_store/Si...p2013.html ?2.39
http://www.amazon.co.uk/exec/obidos/sear...ss=default