pinnerite wrote:
Now (soon) I shall need to buy new. It looks as though 18 x 600 x 2400 is
the default today.

You can get 'loft' boards, which are handy if access is tight, but doesn't
address your immediate problem:
https://www.wickes.co.uk/Wickes-Chip...-of-3/p/116420

The thing is I always aligned board edges along the centre of the joists.
So that would require joists at 600mm.

I think traditionally you lay the boards at right angles to the joists, so
your 2400mm covers 6x 400mm-spaced joists. The joins are therefore at
joists where they can be screwed down.

As mentioned, it's worth thinking about putting in some extra timber to
raise the level and bump up the insulation while you're there.

Theo