Thank you, guys. Some helpful and informative comments above.
Tis interesting that a weak battery can, it seems, bring on the sensor errors (and it's annoying, as I believe there is a separate code for low batt').
I'll search in the garage for an old 9v transformer and my soldering iron.
I think that perhaps there's some truth in the theory that Chrony sensors do need 'conditioning' to the light, after being stored away folded up in the dark for an extended period.
Bless you, Mick.