Glorified storage

It depends on what height you have. If possible, you would probably be better off ignoring the existing ceiling joists altogether. Fix a new floor to the loft with new joists slightly above the existing ceiling joists. These can be secured to joist hangers on the walls or, if you wish, fit angle iron along the two facing walls and rest the new joists on top of this (ensuring you have plenty of noggins to stabilise the joists. If you have a solid brick/block wall in the floor below which intersects the loft space, you can use this to rest the joists on - either bearing on additional bricks to raise above the existing ceiling joists, or resting on spacers connected to the ceiling joists where they rest on the brick wall. If you have no solid wall to take the load of the new joists, you will need to consider an RSJ run across the centre from one wall to the other, and then connect the new joists (3m lengths) into the centre of the RSJ - again, not forgetting the noggins, and noting that this will raise the top of the RSJ above the level of the new joists, unless you trim the end of the new beams so that they finish flush with the top of the RSJ. 18mm T&G would be fine, and C16 would be okay, although I always tend to use C24 for the added strength, albeit that the cost is obviously higher. I would then put plenty of rockwool insulation between the new joists and the ceiling (if there is no existing insulation) or the top of the existing ceiling joists (if there is some insulation already there). The latter makes a difference for both heat and noise separation from the bedroom/s below. Hope this helps. I would recommend 6"x 2" at 400mm centres. This will prevent bouncing/movement of the floor. However, if height does not allow, you could get by with 4" x 2", C24.

For simple storage use I would run timbers across your original trusses at 600mm centres then use 18mm chipboard flooring should be ample and spread the weight evenly.

Employ a structural engineer to make the correct decision