One advertised solution is something that sounds/spells like "no more ply"
AFAIK it is 6mm cement fibre board in 2'x3' sheets that are fixed with
polyurethane glue and screws and cost £10? /sheet.
I wonder if its the same as "RCM Multipupose fibre cement board - A high
performance cellulose fibre cement sheet which can be used in an extensive
range of application both internally and externally. Key Benefits:
Excellent weathering properties, high tensile strength, high MoE, smooth
finish for paint, excellent acoustic
performance. A very strong versatile board." A 6mm 8x4 sheet being £15
on their price list at
http://www.rcmltd.biz/RCM%20Price%20...Jan%202009.pdf. This board
should address water issues but also glued & screwed should add
significant rigidity to a chipboard floor.

Peterk
Is that flexible enough not to crack when put down on a floor? I'm thinking
particularly of floors in old properties where there may be a bit of
movement in the odd joist.