@jcogdell thank you very much for this.
Pick radius fixed it, but I will be trying the NURBS to make it easier on the rest of the guys helping with my files.
In case anyone comes across this while having the same problem, the immediate fix if you're in a real rush is Insert on centre line > move 3D > move by increments of half your truss width in whatever direction;) NOT recommended good practice, obviously, but if the fussing around is the difference to your getting the plot to the riggers in time, don't be shy. I just did that for a week.