Cylinder in loft, first floor walls not on load bearing supports

[Bruce]

We are going to put the new unvented cylinder in the loft to save space on the first floor. The first floor walls, especially the one between our two larger bedrooms are not built with load bearing support right under them on the ground floor, instead they are built on the floorboards that are supported on joists (brown in my drawing) that in their turn are resting on the external walls and beams (beams are blu in my drawing). The wall between the bedrooms is the red one, I have drawn it on the ground floor as well to show that it sits between the beams. The walls on the first floor are built of medium blocks/aircrete blocks.

I cannot put the cylinder on top of the wall that runs in the middle of the house as the roof support is in the middle so it would have to sit to the side where there is not enough head clearance in loft. I think the best place is on top of the wall between the bedrooms. I have drawn my platform for the cylinder in yellow, my idea is to add 3 more 100x50mm beams/(joists?) next to the one that the roof supports are resting on (shown in my photo) and then put a 25mm marine plywood sheet on top of them to create a platform. The cylinders we are looking weigh about 260kg when full.

What are your thoughts, if this good or should I do something different?