For anyone else who has this problem:
There are 2 microswitches to sense the bonnet unlock and they are positioned at the bottom of the near and off-side catches (not the central secondary catch.
My problem was that one of the catches had stuck open, so when the bonnet was shut it wasn't really holding anything down on that side. It was just making the contact though, hence the intermittent failures.
Solution was to lube the locking arm so it could freely engage.
CBA
Thanks again to Revo Ben for his diagnostic help.
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