It was going so well then D12 / D13 came along

[Enoch]

Hi folks, well my build has progressed rather well but today I have hit a potential brick wall. From the book it appears that when D12 is installed correctly at a 44 degree angle to match up with CP7 it should meet the rear suspension frame at the top rear corner, where it joins to D13 - the other end of which joins the bottom bent tube RP1 (or RP2 - can't remember) Anyways when I do this D13 seems to be a good 1/2 inch or more too long, even when the end has been radiused to join RP1. The tube is cut to the right length, the check height of 112mm is correct within 1mm, all other dimensions check out to within a mil or two. Anyone else experienced this? Any thoughts or words of encouragement will be very much appreciated.
Enoch.