So, I need to replace the whole lot and across the whole span. There will be no other interior fittings so no option to do it in two halves.
What I was thinking of doing was replacing the shelf with a torsion box design made from 2x2 and faced with 9mm MDF. Should make a shelf about 65mm thick. Then hang the rail under this. Centre of the shelf would be three strips of 2x2 (front middle (for the rail to screw brackets into), and rear. Then front-back pieces on 400mm spacing.
Has anyone tried this? Would it be likely to have the stiffness that I need over that length? Is it over-engineered and is there a simpler way of doing it?
Any comments appreciated.