Thanks for all your research Brenda. I have just looked at the M&S one and you're right it's 42% Lycra.
I have the Julie Cole book on pattern drafting for knits. In the section on underwear she suggests Powernet in 2way stretch heavy weight. On the M&S one there is stretch width ways, but only minimal stretch length ways. I've just done a stretch test and it's 100% stretch but it does take an effort to pull, hence why it holds you in I suppose. The bra section is a much lighter fabric with the same stretch, which doesn't squash you, so you retain the shape of your bust.
When drafting her slopers /blocks, she reduces in 1/2" increments, so from a minimal stretch i.e. Interlock or ponte Roma to super stretchy, with 100% or more stretch there would be a decrease of 1.5 inches on each side seam!
So you're right the more Lycra the greater the stretch and recovery but it's the weight that gives the strength. I think I will order some swatches, that way I will be able do the stretch test and feel the weight.
Many thanks for all your input, I will report back when I've got some swatches.