Hi guys...Ill get some into try now that its easily available, but do you find that 400 twill is twice as strong as two 200 twill layers. I'm hoping its just as drapeable and the weave is more stable than the 200 twills.
Cheers...Chris

Generally speaking in broad brush comparison a 400g twill made with the same fibres will be pretty similar to two layers of a 200g twill in strength.  Its slightly more complicated when you get down to precise figures but for most non-structural and semi-structural applications it is fine. 

Being a heavier weight cloth, the 400g is unlikely to drape as well as a 200g cloth.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

In terms of stability, a 240/245g twill is usually much more stable than 200g. 

It's usually more tightly woven and the added expense is usually worth it where cosmetic appearance is important.