Yay!! My own fettlage has been successful. Newish 44/34 chainset, new chain and new sprockets, as the old ones skipped like anything with a new chain on.
My Brompton now has 12 gears, with 88" 71" 56" 46" 36" 29" on the 44 ring, and 68" 55" 43" 35" 28" 22" on the 34 ring. This compares with the standard gearing, which is 100" 81" 64" 52" 41" 33". I never needed the 100" gear. So I've now got a general knocking-about set that I can spin up to about koff-koff mph if I have a downhill or tailwind, and a set for touring or climbing. Anything I can't climb on a 22 inch gear I shall walk or get a bus.Or possibly a chairlift,as I have plotted for a putative tour of the Alps including an ascent of the Col de Mont-Cenis.