I finally got round to trying a Shimano octalink setup and things worked out perfectly. Unlike in the article mentioned above I opted for a spacing based on a triple BB, i.e. 118.5mm, and put the chainring on the inside. Chainline is perfect and Q-factor should be around 154mm, perhaps a shade narrower than the Brompton crank. Clearance over the rear triangle is fine with 172.5mm cranks.
The bike is dusty and the chainring an odd colour, so the pics don't look fantastic but they should give a clear indication of the fit.
This spacing should work on earlier square taper Shimano cranks from the 5 years or so before they switched to the octalink type as in that period their chainline/q-factor dimensions stabilized. A square taper system would be preferable in terms of BB choice; road Octalink BBs are expensive now and the Ultegra range has been discontinued. The 105 BB is overpriced.








I've been led to believe that a Sugino XD2 with either a 110mm or 113mm BB would fit the Brompton. This seems like a good option, it's a nice-looking Crankset and people might have spares left over from Dahon bikes or other builds. However I'm not sure what the resultant q-factor would be other than that it would fit the bike.