Tui Hat Wearing Son - thanks very much for the reply.
I thought as much, that it's positive HYD pressure that keeps it up. I have a sneaky suspicion that the gravity extension handle effectively "pulls the plug" on the hydraulic pressure and releases it, dropping the gear. Makes sense I would of thought.
I think that hooks are only used for downlock to keep the gear extended.
Although, I'm in the same boat as you, I have been known to be wrong before!
Regards,
l5otg