Question Author
Robinia - tried that I'm afraid but no joy as it's a cheap machine from Lidl (Lervia is the make). Tried general 'sewing machine tension' tips which is how I've got the improvement so far but I can't work out from the stitching what the actual problem is. That's why I thought a pic might help.
Goodsoulette - I *think* the upper thread is too loose but wanted a 2nd opinion from someone who, unlike me, knows sewing machines. The top and bottom threads are the same type.