As is usually the case it was worse than I initially suspected!
I have now removed all of the floorboards downstairs, which was a bugger of a job. I tried to remove them whole without damaging them so that I would have the option of relaying them in the hallway and dining room rather than paying for laminate flooring, but in the end I ended up taking my circular saw to them [I have no central heating or gas pipes to worry about and I turned off the electrics as well, just in case] - chopping them into nice 12 inch long sections which I could pull off the joists with ease! So the chipboard decision has been well and truly made!!
Once the floorboards were removed I could see that a number of the joists in one room were almost completely eaten through, to the point that on their own they could barely take my weight! In the hallway they were also well on their way to being eaten through, only one room appears to have sound joists, however they share the same holes in the internal wall as the hallway joists so I wouldn't be surprised if the ends are infested once I pull them out of the walls. As such I have decided to make sure and replace all the joists with new.
So far I have cleared the floor space of all rubble [nearly a skip full of bricks and rotted wood offcuts] and cleared out the blocked up air bricks to improve the airflow and hopefully stop the wet rot returning. I will replace the DPC on top of the small supporting wall that runs down the center of each room under the floor and I will insert a peice of DPC under each joist in the holes in the walls [the current DPC is made of slate and looks OK under each joist, there is no signs of damp above the DPC, but a little extra cant hurt I suppose?].
This seems to be a big job and I want to get it right and avoid the little rotters coming back. Do I need to treat the ground and bricks to kill off any stray little critters, if so what would be the best thing to use? Can you purchase pre-treated floor joists that are treated against wet rot/dry rot and wood boring insects? - I must be honest and admit that I have not looked into this yet, I'm currently tied up at work so I'd like to seek the advice of anyone that knows better and likes to share their knowledge.
Anyway, any advice on how to proceed with this job would be greatly appreciated.