I was aware that the B10A had IPv6 bugs, which is why I'm using the HG612 modem/B10D router combination. I'd hoped that as I was wanting to use it as a dumb Wireless Access Point (and switch) the IPv6 bugs would be irrelevant.
Doing an ipconfig /all on the desktop PC it looks like the B10D is advertising itself as an IPv6 gateway which, as it is closer on the LAN that the real gateway which is I suspect why it breaks the IPv6 on the desktop whose IPv6 traffic shouldn't be going anywhere near it.
NAT is I think a red herring as the IPv4 is working correctly.