Some time back I bought the freehold of the building where I lived, which was divided into two purpose-built flats. I can't remember the detail but it seemed pretty straightforward at the time, we had to describe who was responsible for the roof and drains so there was no argument, (top floor roof, ground floor drains), each maintained own external windows and doors. I would think that the council would use their own internal legal department so you'd need to appoint your own.