Hi, I never had slippage on my new brompton - but am not heavy. In the brompton talk group there is seatpost-slippage reported for older bromptons with the old clamp style - so if you consider a used brompton, be sure to upgrade it with the new clamp. What also is reported that the slippage normally stops after a few month, so perhaps also on your bike it needs more braking in time?
Sorry for off topic: where to look at the steel swift?