AdamP

Hi, I have bought property recently and my neighbour has rear garden extension before I bought property. I don't have party wall agreement from my previous owner(although I have requested for it). My neighbour has built the wall over the boundary line for their extension. Now, I want to do my rear garden extension and I requested to use party wall but they said that I have to leave 5cm gap from their wall for my rear garden extension. If I leave gap then I will loose 35cm(5cm + 30cm for new wall). So, my questions are 1. Can I use existing wall(neighbour constructed wall on the boundary line)? 2. How do I find party wall agreement if I don't get it from previous owner? 3. What are my rights in this case?