Nightjar?

I was fascinated by the description of the possible Nightjar. Obviously I'm no expert but it did sound very much like it was to me at least. I really can't think what else it could be. Your description of the bird itself kind of fitted the bill. As for the amazing flying behaviour-I haven't the faintest idea whether they do this, it sounds incredible. I vaguely recall seeing a photo of a juvenile Nightjar once somewhere resting along somebodys garden fence (as opposed to across it), and this reminded me of your description of the bird resting along the apex of the greenhouse. Perhaps Nightjars always perch this way or at least they commonly do so I believe.