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Wak, I did what you suggested, darkened the room, turned telly etc off so it was quiet, left back door wide open and shone a torch up the chimney. A bit of a flutter, and a big fat pigeon landed in the hearth. It was well scared, and so was I. Instead of heading for the open back door, it started flying around everywhere banging into the wall, and I ran out.
My son came, and by then the pigeon was in a corner of the room, on the floor, not far from the chimney. We opened a large window just above where it was and my son got it up and out of the window with the plastic garden tool for clearing the leaves. I stayed well away. Thank goodness its gone - it was quite scary really. I think I was just as scared as the pigeon !!