Thanks all for the comments.
I find many big battles go to ties because of the time involved and space limitations. Not sure about you but I find it frustrating. Still, looks like a good time.
Yes, it was a bit of a pity not to have a winner after 6 hours of play - but it was fun nonetheless.
Another thing that seems to make it harder to get a winner in big games is the fact that the BTS is usually something HUGE and therefore very hard to kill. In this game the BTSs were a Warlord titan and an ork warband with 26 units in hard cover. Neither looked at all vulnerable.
We made a mistake (as the orks) by putting too much in the middle. We got the blitz, but didn't have enough to really challenge for many of the other objectives.
The scenario itself may have made it harder to get a winner. The aim was to make the central blitz the most important thing in the battle, but not the only thing worth fighting for. This didn't quite work, as the marines were able to effectively let us have the blitz and take the fight elsewhere.
I think if I was going to play the scenario again I would keep the tweak about only needing the two objectives in your own half to claim "Defend the Flag" but get rid of the change to "Take and Hold" so that this goal could be claimed by holding the blitz and one other (I promise this isn't because it would have made it easier for the orks to win
This would mean that the central blitz was crucial - if you held that you would have one goal and be close to getting Take and Hold as well. But it's not all or nothing, as the side that doesn't hold the blitz still has a way to win through Defend the Flag and Take and Hold (BTS and They Shall Not Pass as well of course, but they seem hard in big games).