I gave up in the end. I killed the pi a second time as it won't boot, I think, and I can't mend it as I can only work headlessly (I'm in bed). I stupidly removed and reinstalled the fake-hw-clock package, for no reason, trying to fix the horrendous timekeeping. Following that it won't respond to pings.
I managed to set the time, using ntpdate, but it used to drift off by miles in just an hour or so. Even resetting it with an ntpdate every hour wasn't even close to good enough.
I can't use a box that doesn't keep time. So I have to give up on this hardware.
And I have no way of making it boot again now, due to physical limitations, not without hands on the device.
I would need someone to install a fresh headless ssh-speaking raspbian on a good wear-levelling high-speed large (micro ?) sd card for me if I ever regained the strength to start struggling again. With ME, all the effort causes me pain, and so it's just too be a price to pay. And I would just need a known-good, tested recipe that works with ntp, dhcp and ipv6 properly and keeps time. It was a bad mistake when my neighbour accidentally did an install for me on the small, slow card that came with the pi, which possibly would only have a short lifetime with wear rates. I should have made sure that a top card was supplied to him first. Now I'm too embarrassed to go back to my same neighbour yet again asking for further help now I've simply broken it (a second time) out of ignorance, frustration and utter stupidity.
Just really simple things seem too hard like getting ipv6 set up exactly how you want it (it works straight away, but customisation is a fiddle) getting multiple ip addresses per interface going and getting a simultaneous combo of static ip + dhcp + SLAAC + link-local multiple addresses (as I have already on other boxen) and achieving it breaks apps. The ipv6 source address selection algorithm was applied to IPv4 too in Vista iirc. Apps need to get auto-modernised by this, so they can work correctly with multiple ip addresses per interface even with IPv4 if that's the kind of environment you wish to have. For robustness and Lan-local admin to always be easy, I always want to have both link-local and rfc1918 org-scope fixed addresses available on both ipv6 and IPv4, together with global routable ipv6 and IPv4 addresses too, not either/or depending on whether internet connections are up or down or on availability of routers or dhcp servers, and no long timeouts on boot in the absence of dhcp. Achieving that seemed to be messy and screwed ntp, not that it ever worked properly anyway because it was always showing stratum 16 for no reason (no firewall problems) and the box didn't even keep time. I don't know if maybe ipv6 was done in Linux as an add-on or a separate software system, not integrated. I wouldn't have a clue. Microsoft as we know threw all their old code away and started again from scratch, and their superb Vista implementation of ipv6+IPv4 (which took forever to come out) was a single combined total rewrite that benefitted IPv4 hugely, and seemingly has pervasive scope-awareness source-address selection and other good things right in its guts in one place.
So I'm just exhausted and the pi experiment is over. Thanks to all for your help and patience. Just too tired and too embarrassed.