Thankyou so much for all the comments and advice. I've now gone back to the manual and reminded myself how to adjust the pressure foot pressure -duh - however, doing so made the problem worse. I've tried different top tensions, topstitch and quilting needles 11 and 14, 2 bobbin cases with different tensions, regular and walking foot, and I still can't get the decorative stitch to come out right on the quilt, though on the practice piece nearly every combination is OK!
Three things occur to me:
I'm not at all sure how I know when the tension is correct when using these stitches: the straight stitches in the row of stitching look fine top and bottom most of the time. However, I think that my rows seem to be pulling the quilt quite tight, so maybe I need a much lower tension.
The thread - although my machine does usually cope quite happily with the 50wt cotton thread that I'm using, maybe for quilting with these stitches I'd be better with a finer polyester?
The simplest of the 3 stitches I've tried - an outline of a star with no infilling - IS working OK. I wonder if the problem with the more complex stitches is with it being a log cabin block, plus the tightness of the stitching mentioned above - maybe the number and density of the stitching pulls the fabric in and there isn't enough "give" in the narrow strip I'm sewing down to be ablee to do this successfully?
Just thinking aloud here, and would be interested to hear your comments.
I'm actually enjoying this, rather than getting frustrated -the wall hanging isn't precious to me, so I'm pretty relaxed about using it as a learning exercise!