The standard Brompton crank is made by Stronglight and the chainring is swaged directly to the RH crank arm. This is a cheap way to make it (although a 54T crankset is still about 60 GBP to buy), but the disadvantage is that strong riders can rip the assembly apart under starting torque:
This isn't mine, thank goodness - this is the sort of failure that really leaves you stranded.
So I decided to get a proper forged crank with a bolt-on chainring. The Sugino XD is cheap, gives the correct chainline with a short 103mm bottom bracket and looks neat.
However, there is a problem. Most cranks have too narrow a Q-factor (that's the distance between the pedals). If you put a straight edge on the back of the chainring it will usually be about 15mm behind the pedal eye. The Brompton crank is much more "sticky-out" and this dimension is about 25mm. If you go too narrow, the crank hits the folded rear frame and also reduces the already-perilous heel clearance at the rear rollers.
It looks as if the Brompton crank is going to have to go back on for now. This also means reusing the nasty FAG bottom bracket with possible the worst removal tool known to Man.