I think we've found a solution! This poor wee cat is so nervous of closed spaces that she seems to be sleeping under a neighbours decking at night. She is around our house all day but goes over there at night. I tried the "cat and box" trick to give her some shelter and it seems to have worked. The box is under a wooden bench which is under an arbour with a roof at the bottom of our garden. She has been sleeping there for a few hours now.