As Alanl says, it’s 3 core and earth between the two switches. This is flat, like 2 core, and you can use either 1.0mm2 or 1.5mm2 – either is satisfactory.
The colours of 3 core and earth are different to twin and earth, which will be red and black for old cable and brown and blue for new.
(red/brown are live and black/blue neutral). In 3 core you’ll have brown, black and grey.
Wire as follows:
Fron the fused spur it will be red/brown to COM on switch 1. and black/blue into the switch box for switch 1 (no connections yet).
Three core:
brown from L1 of switch 1 to L1 of switch 2.
grey from L2 of switch 1 to L2 of switch 2.
black from neutral in switch box 1 to switch box 2 (use choc. block connectors)
At switch box 2:
From COM in switch 2 connect red/brown of existing wiring to lamp.
From black in switch 2 to black/blue of existing wiring to lamp using a choc. block to make the connection
Mark the grey of the 3 core with brown tape or sheathing to identify it as a live conductor.
Mark the black of the 3 core with blue tape or sheathing to identify it as a neutral conductor.
Ensure the earths are connected from the FCU through to the lamp fitting. The earth in the cables will not have insulation and you’ll need to use yellow/green sheathing to protect and identify it.
Ensure the FCU has a 3 amp fuse in it.
Robert is now officially your father’s brother.
Rich.
PS lighting circuits are almost always radials, not rings. The FCU could be a spur off a ring and as the MCB protecting the ring is probably going to be a 32A one it will be too big to adequately protect the smaller cables of the lighting circuit. That’s why the FCU fuse needs to be 3A. (You’d actually be ok with a 13A fuse as 1.0mm2 cable will carry 16A if it’s clipped to joists but your lighting load will be minimal).