Thanks! Following inspiration from an earlier thread, I got in touch with Ben at Kinetics, and he had a rear triangle he had just taken off as part of a new bike build, which he was happy to sell. Someone on ebay was also selling the reamer kit (a piloted one, with a short cutting section). I heated the countersunk screws, they came out without too much difficulty (using the trick recommended by Kinetics of jamming in a long 5mm allen key in the side where the first screw has been removed). Interestingly, Brompton specifically specify a low strength threadlocker (Like purple loctite 221/222). Now all back together, and feels good. The only frame specific parts I had to change from the old rear triangle were the anti-rotation washers for the hub, but fortunately I had the new style (tabbed steel) ones from a previous hub swap!