Thanks again. Just to let everyone know, it all went fine. Got a spare nut off the nice man at Two Wheels Good in Crouch End who as luck would have it had just replaced a headset and was kind enough to root around in the bin for a couple of minutes, then give me two minutes advice on how to get the length just right (wouldn't take a penny), measured carefully, used the new nut and the one on my headset to create a hacksaw-blade sized gap at just the right point, hacksawed thru' the gap, add 45 seconds with a bit of emery paper, that was it. Done. Perfect.