Cross trainer lack of tension?

I have a Tesco One Body Cross Trainer which appears to be broken. There seems to be no tension when using it so the pedals spin too freely. I've tried adjusting the tension but turning the knob doesn't do anything. I've had a look at the cable but can't find anything wrong. I've had it about 18 months now so it's out of warranty and it was working perfectly until recently. Has anybody else had the same problem with one of these?