[T] It's not 'strength' but 'stiffness' you need here though isn't it.
I think 3 spans would be sufficient as the rear and sides are solid. I
might put two near the front (where there is least supoport now) and
one down the middle.

Yes, that sounds sensible.

Do you think that 3 spans of 19mm box section would not be sufficient
support? If not, would the 25mm push it into acceptable territoriy?

[T] I think the 'stiffness' goes up exponentially with the box
dimensions (that a guess not a scientific comment) so go as large as
you can.

Well, I'll try to get the 25mm in, although I can't be sure it will fit. It
would just be a case of deciding on a suitable really thin soft material to
lay over the top. Do you think a bead of dry sealant will be enough to
prevent the granite smashing due to the hardness?

Christian.