I have successfully upgraded my 3 speed brompton to 6 speedby fitting a double chainring,a new b/bracket spindle and a chain tensioner..Some engineering is required ie,brazing and silver soldering.A front changer fitted on a bracket made up from a 28mmx22mm copper reducing fitting ,although a piece of seat tube cut to size will do.These parts are brazed onto a bracket incorporating the parts and the made into a clip to fit onto the tube below the seat pin.The chain tensioner is a jockey wheel fitted to a bracket on the rear folding frame and secured via a 4mm bolt.The gear and brake arrangement can be via a combination brake and gear lever or a twist grip.Two version have been made and one has been tested on a 25 mile ride without any problems.I will endeavour to put the photos on to this site soon.