Fitted new batteries today & I'm pleased to say it's working perfectly. Shade, sensitivity & delay knobs all doing what they should. I can't get over how clear the view is, I think this is the first mask I've used where I can see the weld pool properly!
I did a back to back test with my old mask with both set to shade 11. I was struggling to see the arc with the old mask and afterwards I could still see the arc when I blinked my eyes so the mask isn't doing its job properly. I had none of these problems with the new mask.
@HughF - we were both right about the sensors. There are 4 on the lens unit but 2 are covered over when it's fitted to the mask! Maybe this is not the original lens unit for this mask? Doesn't matter anyway as I modified the mask so that all 4 sensors are visible.