It's been suggested a few times on this thread that the top part of Manor Mount be made two-way. The problem is that this is how it was for decades, and what happened was that a massive traffic jam built up during the evening rush hour. It went from the South Circular at the bottom of David's Road right back to at least half-way up Manor Mount, for three hours every evening. This was a nightmare for everyone, and even worse for the ambulance, which was often delayed getting out. After years of this the no-entry was finally put in.

The specialised speed humps which do not affect broader-based vehicles like ambulances (and, I assume, lorries) might help with slowing down the cars coming down illegally from the top. The ambulance service always opposes speed humps as a matter of principle, however, and that's why there aren't any in Manor Mount or David's Road. We need them down here as well.

The real solution (apart from the claymore mines suggested earlier, which seem an excellent suggestion) is for the police to do their job. If you got a fine every few months you'd soon stop.