We use The Dogfather for ad hoc walks - the service is very flexible which is great for us as we don't need regular slots every week. Dom and his team take the dogs for 'proper' walks to Wimbledon and Richmond Park and sometimes further afield so it's very different from plodding around Tooting or Wandsworth common on lead .. My dog is always delighted to see Dom if we bump into him out on a walk which is reassuring!
http://www.thedogfatheruk.com
We also saw Corinne (Barking Mad) for a couple of one-to-one sessions and she was great, really helped iron out some minor issues we were having