Hello, Malcolm.
Threading the wire through shouldn't present much of a problem. Have you removed the triangulat step inside the cabin? Below that you'll find a small sheave over which the wire runs then drops down through a hole into the case. Check the sheave is free to spin and not worn. When I got Falcon that sheave was jammed and the wire was on its way to sawing through it - there should be a post on here with photographs.
The crimp on the c/plate end of the wire should drop into its slot in the plate like a bicyle brake cable does. Its slot is accessible through the circular hatch in the starboard side of the case. It is above the waterline
As to diameter and length, I would suggest a replacement from Nick ay Cape Cutter, Driffield. From memory I think the wire was 4mm but it's worth double checking. I don't recall any problems with the drum fitting as there were always 3 or 4 turns left when the plate was fully down. Unfortunately I can't check for you as Falcon now lives in South Australia
Perhaps someone else will be along shortly with more help.
Steve