Hi again Lloyd - thanks for your time trying to help me. I really don't think the video has been overwritten. In any case, if it had been, I would have thought I would see a black screen. It is the normal bright blue colour. This really is a mystery.
I have looked on the internet and see what you mean about costs charged by companies who claim to be able to retrieve data.
If I do find a way of retrieving my footage, I will let you know! In the meantime, thanks again and best wishes.
Joan