cheers everyone, I THINK I've cracked it, I've done one sideframe in two parts and treated each bend as one angle rather than getting confused with two angles, i've then assembled the two halfs and mirrored the sideframe as another component. by making sure the ends of the sideframes meet just over 40mm apart at the downtube i've ended up with what i think is the correct shape, though I'm not sure how accurate it is it looks about right
Abba i've never used autocad so have not got a clue
So, anybody know if you can rapid prototype from an assembly?