Hence
the query about how good, or not, spiral or disc type ground anchors
are.

My current idea is for a ground anchor at each corner connected
directly to both frame corner pieces with an eye bolt and turnbuckle.

I had something similar in January; one spiral ground anchor pulled right
out, admittedly a larger structure (12 * 18 horse "field shelter" with an
open front. The other corner was anchored to an open loop of half inch rebar
into concrete which it straightened out. On doing the sums to British
Standard afterwards (horses, stable doors) I discovered that I had about a 3
ton vertical lifting force for the 20 year gale. Because I *really* don't
want this to happen again (there were two shetlands shut in the floorless
field shelter at the time) I've put a 12 inch by 12 inch concrete filled
trench all round, which is about 3 tons. This has weldmesh in it and rebar
loops attached to about 8 double loops of galvanised 5 mm wire rope, on
turnbuckles, tied in to the roof trusses. That's OTT for you, but don't
underrate the force! I like the idea of a concrete slab floor but I would
worry about the walls and roof leaving the floor behind.