Paul,
I had a thought the other night... if you bring the piers in to a lesser span, you will reduce the bending moments in the joists and bearers. I dismissed this initially because the cost of steel is based on the weight and the saving by going down a size isn't going to be a lot. But if you are scratching for money, it might be worth another run through the calcs with say 2.4m span and 300mm cantilever (over hang) all round? Maybe even a 450mm cantilever?
If you've already bought your steel it still might be worth doing this just to make your structure a bit stiffer and stronger... just a thought.
Al.