Armed with the advice of clever friends (thank you Marianne and Helen, who has altogether far more important things to worry about right now....) I repaired the broken warp thread. Actually, I took the weaving right out, went right back to the start and tied in a new thread, because I hadn't got very far. I started again, more loosely this time, and established more of a rhythm though I really need to sort out my posture before my back breaks!
Clearly things are meant to be looser. I've relaxed and allowed it to take its own shape, which sadly means there are no longer spots forming in the pattern. But the resultant weave is more regular (except for the odd blip like in the photo) and in working like this, I now realise how far I was forcing this when I began.
I also notice that the edges are straighter than they were - and remembered Miss Lin's stern face because I had a tendency to pull it too tight when I was there, too.
Yet another lesson learned - my goodness, this will look good on the CPD* record?!
* Continuing Professional Development, of which I am obliged to do ten hours per year. Half of it must be professional training and the other half can be subject-specific, like this. I've counted three hours of crochet classes so far and will count two hours of learning to weave. Add that to the fifteen hours of professional training done so far this academic year and I think I'm out of the woods, don't you?