There were 2 fairly large unit of Daemonettes, a Herald and a Keep of Secrets. Herald and 1 unit decided to outflank while the rest started on the table. One thing I learnt was that Daemonettes are really, REALLY fast.
In the end it came down to the above picture, which was taken right after the Daemonettes charged me. It took them a while and cost them a marine but the Iron Hands killed all of the Daemon scum bar 1 stubborn crab-ostrich-lady-thing whose survival ensured the Keeper of Secrets got into the combat and started munching on my poor marines.
From what I can tell is my army can't deal with numbers backed up by something big. In this case Daemons and a Keeper, in the previous case Firewarriors and a Riptide. Maybe next time I'll start the marines on the ground and use the pods as cover/road blocks to try and divert them into bottlenecks for the thunderfire.