Thanks guys. I've filled the gap with a sheet of ply for the time being, at least until the weather improves.
In answer to the questions, I'm in Cheltenham, the house is 1960s, don't think its ex council. I think its cement board but could be asbestos, don't know tbh. Didn't drill through anything just in case. Don't have any pics, will take some when it stops raining..