Recently, my derailleur started jumping on the small sprocket. Obviously I thought it might be wear, except that there was a very sudden start to the problem, after which it happened throughout every pedal stroke.
I've just taken a look, after buying spares in case. Seems as though some foil, or maybe some paper (hard to tell) wrapped itself around the sprocket and the channel between the two sprockets.
After spending a while trying to get it out, and finding it still stuck, I ended up stripping the whole lot off the hub and cleaning it completely. I think someone (Kim?) remarked that Bromptons suffer from having low gears - as in gears low down near the ground
I'll keep the spares for another time.
