Ok, so good news.
After torching it, and doing all sorts of stuff which didn't work (at first) I took the extensions out of the mix and just had the socket on the impact and it came right off. No idea why it wouldn't break loose with 200ft/lbs being applied but it's off now.
Currently got the new belt on, but not much else re-assembled.
1991 Nissan 240SX (aka 180SX) SR20DET swap, upgraded turbo, HKS stage 2 cams
2014 Ford Fiesta ST (5-door) Mostly stock
1991 Honda Beat (mods from previous owner - coilovers, cat-back exhaust, stickers/emblems everywhere)