Hi. I went through this exact process with my neighbour's house, and found that I needed to go through a property management company called Amicus Horizon. They're a huge housing organisation that manage Southwark's property. Whether they manage ALL Southwark's houses I couldn't tell you, but certainly worth getting in touch with them to see if your neighbour's house is one of theirs too. Failing that, go through Land Registry to get the official title deeds, which will tell you who owns it. This was how I came across Amicus Horizon, after wasting weeks trying to get the council to respond. Turns out I didn't need to go through Southwark at all, even though it was a council property. (Another important reason to get the official title deeds - for both adjacent houses, assuming you're getting party wall agreements from both sides - is that you have to make sure the agreement has the exact correct name on otherwise some tricksy lawyer will tell you it's invalid if you do ever have a dispute.) It's such a fiddly, boring, time-consuming and expensive piece of bullsh*t legalese, so I don't envy you going through this, but stick at it and you'll get it sorted! Adam