You wont damage the door, you use the hacksaw blade to cut the bolt off in the gap between door and frame. If there is not much gap then try pushing a large screwdriver or chisel in the gap to make it a little wider. There will be plenty of give in the door/frame without it breaking. When the joiner turns up and only has to fir a new latch he wont be complaining that you made his job easier.
Another daft thought, is your consumer unit/fuse box under the stairs? If so you could always make a breaker trip out so you will then be an emergency
