Just chucking this up here as a little aide-mémoire for myself for next time I get bitten by this.

When a network service (in my case it was pdns-recursor) fails to start at boot time, with an "unable to bind to address" type error, but starts manually no problem, double check that /etc/network/interfaces has "auto eth0" rather than "allow-hotplug eth0"

And never work with animals, children, or computers ;-)