Thanks guys - having watched a couple of lockout incidents on You Tube I think I understand the mechanism now. It appears to result from the winch cable being attached a long way below the wing (i.e. to the pilot) and if the wing goes divergent the sideways pull on the pilot then causes the situation to rapidly worsen. Parasails are presumably worse than flexwing hang gliders because of the pilot being even further from the wing.
Note to self:- stick to sailplanes