BJS, you've come through once again! The Cowan chuck is exactly what I want, found I could get one here for about £25 plus £26 delivery from the US but I have someone in the US right now so he's bringing me one back ($25) next week.
Seems one of the problems with them is the chuck tightens up if you put too much pressure on it when drilling concrete etc so found a video by the inventor who shows the right way to release the chuck if it does.https://www.youtube.com/watch?v=I7MQf1s2ys4
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