Thankyou both for your advice. I'm afraid its actualy broken -I've spent a long while removing the plastic covers and part of the upholstery to reveal the mechanism and I can see that when I pull the tab on the left of the seat (as seen from behind) the left hand half of the mechanism releases but the right hand half doesn't move. If I force the right hand half with a screwdrive and apull the tab then the seat folds (but it needs three hands!). There's a rod that links the two which is not attached to the lever. There's a four pronged piece of metal that folds back onto the base of the lever and I guess should be welded to it, but at the moment it just revolves freely. I'll try aralditing it to begin with, but it looks like it might need a spot welder which I don't possess.