Hi John,
Good to see you again. :)
8T may seem a lot at first glance, but here's the calculation in detail.
The density for DTp1 is 1.98 T/m3, so.....
30m2 x 100mm = 3 m3 @ 1.98T/m3 = 5.94T
However, the ss allows for spread around the perimeter of, IIRC, 500mm, so we have 22m of perimeter (5x6m = 5+6+5+6) which gives 11m2 extra sub-base....
11m2 x 100mm = 1.1 m3 @ 1.98 m2/T = 2.18T
hence, 5.94 + 2.18 = 8.12T
If you reduce or eliminate the spread, you save quite a bit of stone. :)