Huzzah - today I've cracked the problem with the 'misbehaving' Hemingworth thread.
My first job whenever the stitching isn't perfect on any machine is to change the needle. They are so cheap compared to almost everything else and it's so often overlooked. I buy them in boxes of a hundred at a time. I find I'm not so 'precious' about them when I have plenty in stock.
Well since it already had a new needle (I use a new one for each new project) that couldn't be the problem.
I changed the machine speed from 'normal/fast' to 'snail'. I re-threaded the machine (just in case). I fooled about with the tension and finally, I heavily starched the backing fabric and voilaSuccess:-)
Almost Yarndale Time
45 minutes ago