I heard somewhere that if you are going above a certain speed it is too fast for the Gatso to take a reading. Not sure if it is true.
This is true. It was tested by Top Gear (I think, long time since I saw it) to see if it was myth or reality. It turns out, though, that you need to be exceeding 190mph or thereabouts. So I think the likelihood of you passing a Gatso at that speed is pretty slim, even if your car can theoretically get there (like mine! ).