Hilary,
We had exactly the same problem with a brand new Lunar Lexon last year. There was no central support, and the bed really sagged, and felt like a ten year old Travel Lodge affair. Even without anyone on the bed, when I looked through the access hatch on the outside, it was apparent that the bed frame was bowing under the weight of the matress alone. After complaining to Lunar about that (and a lot of other things) they made a wooden support which was attached to the floor and sat under the centre of the frame. It was actually much better than the metal thing that's attached to our new Lunar Clubman, which is attached to the bed frame and lifts with it.
I would go back to your dealer and request that a centre support is fitted.
Nigel.