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Hi, I need to fit a new light fitting in my downstairs loo. When I took the old one off I noticed an extra yellow cable. I've since found out that this is for the extractor fan, operated by the lighting pull chord. Problem is I now have 4 cables to go in to 3 terminals on the new light.
I understand that the yellow cable is a switched live so does that mean I can put it together with the main live cable into the same terminal

Depends what you mean.

I doubt the yellow wire is the switched live, from here all we can do is guess.
More information is needed to help you, pictures (in focus) would also help.

Light (wires, not the light itself)
Fan
Light switch

Believe it or not there is no such rule that says this wire shalt go there.