Thanks Paul for the word of reassurance, it would be costly shame to give up now. I can see that adding further joists is going to be the way to go, it's a matter of working out the most straightforward method providing enough support. Currently the joists rests on bricks 400mm apart (error earlier, not less) and between each joist sits a concrete block, it appears to be the height of the joist, it's hard to see above the level. I'm trying to think of a way round having to take up the tongue and groove boards, do you think it is possible to somehow jam in an additional joist between the originals, removing the block first? If so would there be sufficient support to span only half the width of the room so it could also rest on the mid-way supporting wall - As long as it could be squeezed under the boards at each wall (or attached to the masonry itself at the outside wall??) this would seem a technically easier way to manouvre to joists into position. I guess if they have to be full length then there's no option but to take up the floorboards.
I do appreciate everyone's help!
