Fair Trading’s much-quoted “Who’s responsible …?” document is ambiguous on this as it says that wiring in a ceiling is common property but wiring in a wall that services only one unit isn’t. Complicating the issue is that many apartments have false or dropped ceilings which, according to my block’s lawyers, are technically a wall.
So I would say that if the wiring is embedded in a concrete ceiling, then it is common property but if it is above a dropped ceiling and only serves that unit, then it is lot property. Make sense?