The secret of this error lies in the winding of the linen warp. I wound half one evening, half the next. It might have just been that one day I was pulling the yarn tighter on the warp board. It could be that there was a second factor - humidity. Linen is stiffer when drier, more flexible when damp.
I knew that there was a difference in tension when I was winding the warp on the loom, it was showing where the warp pulled through the lease sticks
(Super little lease sticks by the way, they came with the loom and you see them here joined together with a treasury tag and tied to the castle)
The warp in front of the lease sticks on the left is distinctly slack, whilst on the right the tension is fairly even. When the warp was wound on the ends on this side were a few inches longer. I trimmed them off before tying the warp on, assuming that the tension problem was resolved.
It looked fine when weaving, and fine when I first removed the cloth from the loom, only showing up when laid out flat to measure the piece before washing.
Something else I discovered very late, only when trying a few inches of wool weft near the end of the warp. A threading error, that showed more with the wool and more on the reverse of the fabric.
Somewhere I have a mirror for inspecting the reverse of the fabric while it is on the loom - next time I should use it! This error hardly showed from the front with the cotton weft (left) although I would have seen it on the reverse (right).