I thought I'd come back to this thread, which I started in some despair a few weeks ago, to let you know what happened. Since then I've been on holiday so haven't had much time to try out all the tips and advice that was offered, but yesterday I took a deep breath and went back to my machine. And - I found a combination of settings, needle, thread and bobbin case that worked! I'd only intended to start to work on it, but by the end of the afternoon I'd stipple-quilted half a quilt - and really enjoyed doing it! The results weren't perfect, but definitely good enough. In 7 years with this machine, it was the first time that I'd had a trouble-free FMQ session. I'm so thrilled - all my enthusiasm is back after a long period of frustration and loss of confidence.
For info, the combination that worked was: no.11 top stitch needle; Superior Threads King Tut thread top and bottom; blue dot bobbin case; tension on 9. So the tension is still a bit extreme, but if it works, that's fine!
A big thankyou to everyone who gave me help and encouragement.