OMG I think it is solved! Someone on a facebook group suggested it might be the heat..at first I thought no, it was switched off for a week when I last tried so it can't be overheating. But with the weather we've been having, I thought to myself, actually even though the machine is kept out of direct sunlight, this is a modern well-insulated flat (as opposed to a draughty stone cottage or whatever) and considering it's over 26 degrees as I type this at nearly 8pm with all the windows open, when I'm out all day with the windows shut i bet it is just really hot all day. The machine is sitting and stewing at at least 25 - maybe the thread is getting just as hot, bothered and sweaty as me! Metaphorically. So I was discussing with a sewing friend at work saying, maybe I will just wait and see if it sorts itself when this heatwave is over. She said, why don't you try putting the thread in the fridge for a bit? Worth a try! So I popped the bobbin in its case, plus a spare full bobbin and a few spools for good measure, in the fridge when I got home and after a couple of hours, decided to see if it made any difference. I have now quilted a baby quilt with no breaks!!! I've popped the threads back in the fridge ready to do the binding another day.
Thanks for all the help and hope this will assist anyone who searches for such things in future!