Hi Stephen

Sink several 10' x 4" posts 2' down into postcreted holes.

I built an 18' x 6' deck for my sister in law nearly 3 years ago. One long
egde and one short edge rest on 6" x 2" timbers fixed to walls. The other
two edges are supported by 4" x 4" posts as you describe.

Then bolt 4"x2" joists to the uprights above ground level (this would
make it far easier to make sure the joists are level).

Exactly what we did, posts at 4' intervals, joists held to them by 8 x 100
coach screws, 3 in each.

This would create a fairly rigid and strong box structure, but mean that
the weight of the deck would be supported by the bolts going sideways
through the joists into the uprights (I suppose I could cut the joists
into the uprights but wondered if this would be necessary).

If you think about it, not only do you have the strength of the bolts
(huge), you have the friction between the timber surfaces as well - 4" x 4".
The deck we built is immensly strong & rigid and hasn't moved a millimeter
in nearly 3 years.

HTH

Dave