Thanks for the help so far. GG - no not using a key scrambler - didnt even know they existed outside of spy films. Mixture , yes I'm on Windows and that's just what seems to be happening - but only on that one key. Spurlash - its on a USB - I'll have a go at your suggestions.
Annoyingly (or not) the problem seems to have gone away for the moment. Reminds me of the smug expression on the face of some chief tech when he wrote "ground tested and found serviceable" in the F700 after some snag you'd entered. You knew he thought you were an idiot who just couldnt use the kit!