Thanks for the advice guys. In the end I used wood to prop up the floor. I used a car jack to take the strain of the joist and then tapped in the wood on a piece of DPC and then let the jack down. It took a bit of work as I found if I made the wood slightly too big it would lift the floor and make its neighbouring pier fall out. Once all the piers were in I screwed them to the joists. I also added noggins right up the middle and where the board edges are.
I also bought some new OSB boards but found the old ones had swollen that much there was too much of a height difference between them so decided not to fit them. So I refitted the old boards upside down and screwed the lot on to the joists.
Its like walking on concrete now ;o) Whereas previously the units with a breakfast bar on it would bounce as someone walked past.
Couple of photos if anyone's interested.