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That's pretty common on lighting circuits, usually the 'extra' wire is used for a strapper in hallway 2 way switching scenarios. It's possible that they only had that wire available and one of your conductors is unused, in this case it would likely be the brown and black carrying line and neutral. You need to test them with an appropriate voltage tester.
It does sound like you are a little bit out of your depth though, don't assume that the colours of conductors mean anything in a lighting scenario. You need to understand and double check everything and work with the power off.
Airhead
@Double_J
So I'm trying to wire in a new external light, outside my front door. There was an existing one when I moved in, but it was dead.
New light wants, live, earth, and neutral, as I'd expect.
Old light had, earth and what looks like 3 core live? brown/grey/black
Help. Do I need to run a neutral wire out to the connection? Strangely there is a neutral wire in the cavity behind the switch (inside) but connected to nothing.