Hi again! Thanks everyone for your very informative replies to this mornings post :-)
I tried everything this afternoon. Threading, re threding. Bobbin all possible ways around. Even took apart bobbin compartment and put it back together! I was so stressed that my hubby (fortunately his day off!) took little man to the park so I could think in peace. I looked at forums, troubleshooting and info on the net. I think my prob was that a little screw where you put your bobbin can be tightened to adjust tension. But I tightened it as much as poss with the little tool they provided, but it was still slack really, and I could see no way of making it tighter. I came to the conclusion that this machine is far too complicated for me!
So before full nervous breakdown occurred I took it back off to Tesco's. The lady remembered from yesterday and said they had taken delivery of some of the simple Tesco machines (same as the one I had taken back yesterday that had jammed) so I have brought another one of those back with me to try out. Oh I do hope it works, as I was really pleased with the simplicity and functions of that machine in the first place (until it broke!) so I got my cash back that I put towards the Singer. I'll let you know how I get on! Can't face it right now though....need a break from sewing machines this evening!