My Mk 2 Brompton, bought used, had a chronic slipping seat post,
lost a half inch or so in a short days trips, new plastic bushing didn't help.
so I went down to the Local Bike shop and got another seat tube Quick Release band
for a 31.8mm seat-tube , they have a Lip edge to keep them on top
That Lip inside, locates on top of bike's frame tube, normally,
Filed that down so the whole thing is 31.8 ID , flipped it over
so now there are 2 Quick releases stacked on top of each other
And the metal on metal grip is, it seems, holding well ..