Brompton uses a BSA (English) standard bottom bracket system. On its drive side (crank side) the bottom bracket is left threaded, which means you have to turn clockwise to unscrew the bottom bracket shell. One of the Brompton DIYer might have mistaken the removal method, he twisted the Brompton plastic bottom bracket in the wrong direction until the cover is totally broken off. Hence, there is no more gripping area for any tools.
The main problem is to remove the left over bit within the Brompton frame, and that is exactly what the Bromptoner came to me for.
We tried a few “lighter” methods and it didn’t work out. It seems the only way out was to build a tool to “pull” the broken bottom bracket out from the Brompton frame.
The following images explain the steps: