Dave Page wrote:
I am planning on insulating my loft to the recommended 270mm, and would
also like to lay floorboards above the insulation so I can keep using
the loft for storage. The gap between the ceiling and the top of the
existing joists is 70mm, so there's 200mm to go.

I'm intending on adding two strips of 40 x 100 on top of the existing
joists, screwing them down, and laying the insulation between them and
boards on top. It seems like a fairly simple job.

When topping up old insulation (Rockwool) it's normal and more effective
to lay the second layer perpendicular to the first. So you'd be better
off fitting the new joists perpendicular to the originals I'd have
thought. However, your original ceiling joists are not very meaty, so
depending on the span, you may have a problem there, if you are going to
pile lots of new joists, floorboards and storage items up there.

There's one bit of information I'm missing; what sort of timber would be
appropriate for this job?

Just bog standard timber joists from the timber yard - haven't a clue
what species it is!

David