Hi, please dont take this as gospel, but i use to play with my brother in our parents loft and it was exactly the same as yours, going by the pictures. My dad and i just boarded it out with old floor boards and i even at one point had some weights up there. You could to make it stronger tho, you could fix a piece of 5 * 2 on the gable walls and double up the ceiling joists to run along the side of the existing ones. you could have the ends of these timbers on jiffy hangers nailed to the timber you bolt to the gable end. This would probably cost you in the region of £100.00 in materials, and would make it very strong. But please note this is not how a loft conversion is done. this involves large steels etc. regards CBM