Nearing the Spaceport Victory part of the game Andy had control of all four of the Spaceports:
Only a daring attack by the Blood Angels managed to capture the Lion's Gate Spaceport and kept the game going.
Eventually, with the Imperial's reserves arriving soon from the warp, Horus managed to trap The Emperor in the Factorum in the south. However, using some Secret Tunnels, he made good his escape and so the Space Wolves and Dark Angels arrived signalling an Imperial victory.
In all I managed to kill 3 of the Traitor's Primarchs for the loss of just the Khan so it was as much a moral victory as it was a hold out. It was quite a tight game all the way through and I'm looking forward to playing as the bad guys next time. I'm not sure how that will go as I'll probably be too aggressive with them but maybe that's what's needed as you can't hang around once the clock is ticking.