Folding/Tressle Ladders

 Just brought an Aluminium muliti purpose ladder to replace old wooden steps. Lots sold  on line for about £65/70. Used it yesterday to wash roof of M/H. PROVED LETHAL. In the trestle position you have two aluminium planks. They just lie on the trestle and move as you do. As I moved towards the centre the plank just tipped up throwing me off the ladder. Fortunately no real injury. For some reason the planks dont fully fit over the tread parts making them unstable and dangerous. Have arranged return as "unfit for purpose " and should be picked up today. Seeking refund. Anyone else had similar experience? The ladder itself looks OK and I would consder another if I could find secure platform. Anyone out there with a safe set.My guess is I will have to pay £100+ for this.