Thanks, both answers as I expected :)
The situation that prompted the query is a new light (addition to existing cct.) centred on a chimney breast that has an ornate cornice above.
The cable cannot go straight down (mantle piece and fireplace in the way) cannot go straight up and furtle behind...
Where there is an ornate plaster cornice (as found in Victorian/Edwardian houses etc.) does the top 150 mm of the wall start at ceiling level or the bottom of the cornice?
Does the front corner of a chimney breast count as the edges of two walls?
Thanks