OK... Seems like my months of correspondance with sky has finally reached somebody who knows what they are talking about. I'll refrain from naming the very helpful chap who phoned me out of the blue yesterday, just in case he gets inundated with support calls, but the upshot is that Sky have given me permission to use a particular work-around for the problem.
Now it doesn't actually fix the bug in their firmware at all, so doesn't really help anybody else who is in the same position, and I'm still waiting to hear about the GPL source code, but at least I can move on and stop worrying about their router upgrading itself to a broken firmware as soon as I reboot it.
Now to move on to getting the OpenWRT box in the middle of the network performing correctly..