[stevehertz]

It's a different model to your link Lawrence, but I think it's just the outward appearance as the the cct - without scrutinising it - seems to be the same. Anyway, there are two diodes on the board as I now see. The detector is indeed in the middle of the board, and the second one (the one I was referring to with the red spot on it) is in the upper right quadrant. I'm not sure what its function is. I'll check both.

If you look on eBay and search 'Spidola' you'll see two types come up.