A few years after purchase, Chain Slipping Off was the only major problem I had with my own 3 speed Brompton as well for the almost 7 years I had have it. I simply slipped back on as I was on the road, not at home. When I went home, I thoroughly cleaned the chain & oiled it again with a little squirt of oil. I examined any part of the drivetrain for damage (there was/is none) & especially the chain tensioner adjustment for possible adjustments and/or damage. There was none there as well. So far, I have no repeats of this problem. If the chain does continue to jump off, I would visit your Brompton dealer.