The issue continues; here's follow-up email I sent PCCW, after dropping them a message to say I still have this trouble:
Just had a technician round to check my modem n home wiring: all ok, which not surprising to me.
My belief: the lightning causes electromagnetic field, which interferes w broadband signal being carried to my home – enough to stop the broadband connection, which then has to restart.
Even with lightning maybe 10km away this can happen.
See this image, from time when my broadband connection was often stopping: you can see there is no lightning record exactly over Cheung Chau, and most is over the sea south of Hong Kong. [I've previously had problems w lightning storms nearer to Shenzhen]
How long is the link from my home to wherever a broadband "router" is on Cheung Chau?
– I reckon the cable carrying it is not protected from electromagnetic fields.
Guess it would be difficult to protect, anyway.
A friend at north Cheung Chau, where I used to live and had same problems, also still has this trouble.
So, perhaps affects places outside the main village area on island.