Thanks for your replies.
Yes, tex, sometimes using something simple is the best idea.
I did in the end find some software called ProfiCAD (free for non-commercial use) which was the sort of thing I was after. See:
http://www.proficad.com/
There is a motor cycle wiring diagram in their gallery which has a lot of symbols on it appropriate for the Roadster.
But for now will study the Westfield diagram, and actually make some progress on the chassis.