A rare visit to the family history centre in Brighton has given me a rough answer to my own question. The city bounadries go from Trafalgar Lane Portslade, include Mile Oak, the farmland around the Hove and Water Hall golf courses, the Pylons, Stanmer Village (but not Falmer), Woodingdean and further farmland until it reaches the part of Saltdean to the west of Longridge Avenue.
The City plan is on two large maps and I found the nearest place of significance to the very centre is the bottom of Hollingdean Dip
