The Brompton left-hand pedal uses a standard 62032 cartridge bearing, which is a single-row, deep groove type. As an engineering solution this is pretty rubbish, since this type of bearing *hates* a bending load and all other pedals have two rows of bearings to support the cantilevered pedal body, but it's how the pedal folds with such little projection and we're stuck with it. Anyway, the bearings don't last very long under hard use.
The problem comes when you have a good pedal - i.e. one that hasn't fragmented from being struck on a kerb - but a shot bearing. You can't just knock out the old bearing and press a new one in, because the plate that carries the bearing has a retaining lip machined after the bearing (a Russian one of unknown quality) is fitted at the factory. This stops the pedal body falling off the bearing, I suppose, but means you either have to (a) buy a new pedal for £35 or (b) buy the carrier plate for an outrageous £20, and fit it yourself - which looks rather tricky.
So, with nothing to lose, I filed off the retaining lip. This is really easy to do because the plate is aluminium and the old bearing is steel - so you know as soon as you've gone far enough and it's virtually impossible to go too far.
Next, the old bearing can be whacked out from behind. Use a socket as a drift to avoid marking the bore. As long as the whole bearing comes out, it doesn't matter in what state it ends up, because it's going straight in the bin. It will require quite astonishing force to shift it if it's been there for a few years.
Now you can clean up your filing with some fine abrasive paper, rounding off the new ridge around the bearing bore so it looks nice and there is no remaining swarf to impede the new bearing.
Finally, take your new bearing (all of £3.69 for a best quality SKF one - you can get a stainless one for £10 if you like, but they're not as robust). Clean the bore and the outside of the bearing with meths, let it dry and then coat both with Loctite #271 bearing fit. Then simply smack the bearing into the pedal using a block of wood to avoid damage, ensuring it starts square. Because the bearing will finish perfectly flush with the body (assuming you didn't overdo it with the abrasive paper) there is no need to use the old bearing to seat it. Allow the Loctite to cure before riding.
It remains to be seen whether the pedal body will work loose. Given how hard it was to shift the old one and the use of Loctite, I very much doubt it, but if it does then a large washer held by four screws (drill and tap the plate) would retain it mechanically. Future bearing changes are, of course, trivially easy now.
