Thanks for the replies. Al's demag tool is shown alternatively magnetising and then demagetising screwdrivers on Youtube
https://www.youtube.com/watch?v=dSHSoIqfZ04, and is available online at prices from about £3 (inc P&P) upwards. I searched Google before starting the thread and didn't seem to find much, but searching Youtube gets a variety of hits including home-made solenoids.
Given that there are times when a magnetised screwdriver could be either desirable or annoying, that particular tool seems quiet useful.
B
edit see also
https://www.youtube.com/watch?v=81Vuhe-ytz4