adding a light on ringmain

I have built a cloak room in my hallway and now wish to add a ceiling light. Because of the awkward position and layout, the easiest option for me would be to power the light from an existing plug socket in the cloakroom. I propose to run the cable through surface mounted trunking as the walls are artexed and i don\\\'t want to damage the surface. Am I correct in thinking I can do this?
Do i connect in to the existing plug socket with 1mm sq flat twin and earth, run it up to a switched fused spur, then on to a ceiling pendant?
Is it as straight forward as this? What size fuse in the switched fused spur?
Many thanks in anticipation of help, wiring diagram would be great

Make sure that the outlet you want to take off is on the ring. You can't spur from a spur.

Yes, you can run the cable in trunking, but you must use 2.5mm2 T & E cable from the socket to the fused connection unit, put a 3A fuse in the FCU, and run from there to your batten lampholder or whatever.

Is your main supply in that room?

If so, you could run a feed (in 1.0mm2 or 1.5mm2 T & E) from the downstairs lighting circuit to an ordinary light switch then on to the light fitting. (No FCU needed)

I am doing the same thing in my conservatory which has no lighting circuit, so I will be fitting a spur from the ring main. You say I need 2.5mm2 T & E from the socket/junction box to FCU but what type of cable do I need from the FCU to the light fitting?

1.0mm2 t&e.

though its perfectly acceptable to keep running to the light with 2.5 if you only wan't to buy one coil/reel of cable

Tried connecting 2.5 into your average domestic light fitting? It's a struggle. Why make life difficult for yourself?