Gah, it was all going so well for a while - until I worked on the middle of the quilt again (I'd worked the very centre, where I had all the initial problems, then worked my out the edge, where I thought I had it sussed, but now I have gone back to the middle to work out to the other side...)
My spools don't have notches on, so I know it is not that.
I'm going quite slowly (DSM, feed dogs down, quilting foot), and I'm fairly confident the tension is correct - the threads seem to be meeting in the middle and don't seem too tight.
But, the thread (Gutermann) is really easy to break by hand, both reels in their different colours. I mean really easy to break - but I only bought it last month. I don't have any other variagated cotton thread (or indeed plain cotton) to compare it to - I've previously only used the Gutermann sew all poly thread.
I have ordered some larger (14) needles online (How I wish I had a wonderful LQS nearby!!).
Thank you all for your expertise!!