Hi Chris,
Just seen your post, had visitors for last 2 hours - comes with the sun i suppose.....
The car is designed around ERW box section and as you say the triangulation gives it its strength. What HandyAndy does is to cut and put together the chassis packs in 2mm walled box, unless otherwise asked, which gives the frame a lot more strength and stiffness for not a great deal of extra weight.
Andrew