Follow up. Saw the lbs this afternoon. They very kindly ripped open a new CatEye cordless and gave me the spacer and a longer screw to get the desired offset from the fork and closer proximity to the wheel. Still no joy though.
Local jewellers confirmed the battery is still 100% ok - it's nice to go somewhere where they don't try and sell you something just because you are in the shop.
Back to the lbs for another try. We fit it onto another Gemini and find that the distance between where we have to fit the sensor and magnet and where the computer fits on the bars is never going to be less than 60cm. Spec range is not greater than 45cm. It works if we hold the computer around the 40cm mark and less. So it seems it is just a distance thing.
Surely there must be another Gemini owner out there who has fitted a computer? If so, what type? Problems?