The wave of new London bike shops

But I'm undecided about 631, which these days you can get on low end frames like IRO etc. Never ridden it so I don't know what it's actually like, but it surely can't be that good if you can get it on such cheap frames..

631 is 853 without the heat-treatment process. It's intended as a TIG-weldable replacement for 531 (manganese-molybdenum). For anything that doesn't need to be really super light/stiff it's ideal. It's not hugely expensive, but neither was 531 which as well all know was the benchmark for awesomeness since before WW2.

See http://reynoldstechnology.biz/our_materials_631.php for details.