......three hours later and with high stress levels I finally produce said cushion. See below for the culprit, it looks OK from a distance but close up non of the points match and one of the central triangles is definitely bigger than the others! And this was after I unpicked it because I made perfect points on the edges only to realise I'd left no seam margins...grrrrrr
So for the moment, I'm sticking to wonky and not getting too bothered by my lack of technical prowess. Here's the latest creation, made with layers of random stripes so none of the seams match!
Maybe I'll try triangles again in 2011 :)