Yeah, I'm stuck with a long rectangular room, I knew that wouldn't be ideal, as long as the fact that the walls aren't covered from floor to ceiling isn't too much of a problem all is good.
I've actually worked out a good way to get the ceiling treated. It involves only one hole going into the ceiling and the rest is using the fittings in the room (door frame + curtain rod) to suspend some covered rockwool. It will hang about 45mm from the ceiling, 1m from ear level at the listening position. The only down side is that I will have to use some really light stuff so it doesn't all fall and fuck up the room or my gear in the process.