Running up a hill
Now I believe I know why there are angles in the architecture of my weaving. It doesn't appear very clearly in this photo but the weaving here is NOT straight.
I read in my library copy of Mary E Black's Key to Weaving that one possible cause for "the edge of the web runing at an angle to the reed of fell" is that "the warp has been tied with a tighter tension on one side of the loom than on the other".
On inspecting my warp this is exactly the case. I am of course delighted to have discovered the cause of my problem, but also disgusted with myself that I didn't get it right in the first place.