Thanks so much everyone...have noted all suggestions and will dutifully try them out.
I'm having a sewing afternoon today so will check my tensions the way you suggested bigsister and see how I go. Yes Fay, the machine was serviced not so long ago and has been sewing fine so I suspect it is my technique and my mucking around with the tension perhaps. Nice to know that the little Janome 360 can possibly manage it if only I can get things right! Hmmm but the lure of a new machine is tempting! Hadn't thought about a private lesson Holice...excellent idea, I'll certainly look into that in the New Year!
Oddly, and I forgot to mention this in my previous post, when I try to do FMQ on a practice swatch the stitching comes out a lot better....could it be that it is just my inexperience in moving the weight/bulk of the quilt that is contributing to the crappy stitching on the back?