So here is my not particularly interesting post:
I have cut the work off the loom and steamed it so that it is closer to a rectangle - it is still a bit wider at the bottom, not enough to worry about.
I had lunch with some tapestry weaving friends today and we discussed this problem I have, as well as the fact that the warp was showing a bit in the tighter areas. The consensus was that I was pulling too hard, especially on one side. One person reported that she had been advised that this often happens in the open shed pass.
In the meantime, I have turned over the edges and made the sample tidier.
I will have to try another sample and see if I can rectify the problem. I think I will get to work on another colour sampler and try to keep it even. My last one was ok for the the colour (in that I can see what I need to do in future) but terrible for the pulling in! I have plenty of warp left beside it, so I think I will give that a go.
I aim to do three types of graduating colour, to see what suits my various purposes best. Who knows if I will ever get the three done - but that is my aim.
However, my next project is in the quilting area, so the tapestry will have to wait a bit longer.