Glad the KISS solution helped ..
I have a six month old Brompton 6 speed with the two speed Sturmey-Archer (I believe) rear hub and 3 speed rear derailleur.
I'm pretty sure the problem is in the two speed rear hub,
no actually the 6 speed is a 3 speed IGH with 2 sprockets
that is shifted by pushing the upper pulley sideways with the shifting mechanisim
a fork pushing the flanges molded into the pulley itself.
2 speed is a different hub just using the 2 sprockets and the chain shover between them.