Ahren - thanks, you've supplied the last piece of the jigsaw(see my signature !).
I've spotted an off the shelf source of perforated steel belt, but couldnt see how to produce the sprocket to drive it. These guys have shown me the method - ball bearings in holes around the face of the drive pulley.
Tape source ? The new digital retractable tapes have holes down the centre at 1cm intervals. Don't know the accuracy, but it will probably be fine for my use.
John
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