Thanks for the suggestions - I think I've got plans A, B and C now!
The frame's continued to soak in penetrating oil for another night so later today I'll give it hell with a pipe wrench. (before toping back up with oil I expect).
Turned into a bigger job that I anticipated but not looking to back down now. Cranks seized to bottom bracket, bottom bracket seized in frame. Penetrating oil + old mtb handlebars attached to huge spanner for leverage + a few hours did the job on those.
The frame is a Glider Tupelo, built by Serotta for the bike shop Cycle Fit in London a few years back. I'll get some pics of mine at some point but here's a nice one from the internet.