Thanks everyone but alas...new needle, rethreaded, changed bobbin and brushed around inside...nup, no fuzz bunny culprits or loose bits of thread, Janome was serviced about 12 months ago and now gets minimal use, lengthened stitch, shortened stitch, whacked up tension, whacked down tension, altered presser foot pressure (as much as the old Janome can as I think it's always been a bit dodge) and lastly turned machine off and then back on. sigh, still the same........starting to look wistfully over at the Bernina who doesn't look as though she wants to get involved in this project at all.......sigh, okay, just for "a laugh" I tried doing a sandwich with flannel pieces instead of the polar fleece. AND guess what!!!!!! The Janome instantly became my friend, sewing perfectly balanced stitches like there was no tomorrow!! So Humpty is going to get a quilt afterall but it is going to be a flannel raggy rather than a posh polar fleece. End of story.
I guess if the old Janome was my only machine I would want to persist to see what it's aversion to the polar fleece is but as I predominantly use the 710 now I'm not going to worry...life is too short, Humpty needs a quilt post haste and I have a possible motorbike purchase looming this morning once this is all done and dusted!
Thanks everyone for your ideas and commiserations, have a nice day.