Alko Wheel Clamps

Has anyone else had problems with the alko wheel clamp? Apart from the cylinders jambing and having to be returned to Alko, the whole thing is difficult to fit and entails jacking up the caravan to align the mounting plate, a nightmare on a twin axle with two to fit. Using the alko jack, the van slipped of and narrowly missed trapping my hands. I am not sure that they are worth the 10% discount on the insurance premium? Any comments?