Thanks very much all for your suggestions. I have managed to screw the back edge temporarily to a piece of 4x2 which has taken the bow out of the back and enabled me to screw and glue the 12mm ply back to it. Hoping that once the glue has dried to full strength (it says 24 hours) and I remove the screws it will stay in place or at least be better than it was. Luckily the other side is ok so have been able to position this one next to a good unit so hoping I can then screw/ bolt it to the good one and keep the bow under control.

Thanks

Lee.