So far we've established black = neutral, red = live, green = earth. We know which 'hole' the relevant wires go in thanks to a useful wiring diagram. What we cannot fathom is what to do with the seemingly spare neutral wire. We have 3 of them with only 2 holes to put them in?

Send tkrussell a message, I am not following you here. Sorry.

Post message here, White or grey should be used for neutral.
You say you have 3 neutrals beside the 3 listed wires?
If it has 2 connections, 1 will be hot from switch and other needs to be neutral.
Put black tape on any neutral that is not white.

What country are you in?

My guess, since there is no answer, is your not located in North America, most likely in the UK.

The device your connecting needs both the black neutral and red hot, at least. The earth may not be needed because the device is all nonmetallic or is double insulated, or the Green earth connection is elsewhere on the fixture.