A kestrel or a sparrowhawk, not sure which.
Big deal you say...
So we were on a treasure hunt, which took us to Buncton Chapel at Wiston. It's a tiny Saxon chapel up on the hill, along a muddy path. As we got near some people passed us and asked if we were going to the church. We explained we were in pursuit of clues. They asked that we leave the door open so that the "big bird" inside could escape.
It was perched high up in the rafters, flying back and forth as I tried to get a picture on my phone, but no joy. Dark upper, barred lower parts, I think. Can't remember the wing shape.
What was most likely the next was outside the chapel, a pile of sticks wedged between a down pipe and a corner of wall, next to an open window, which must have been how it got in.