An update - I've tried the Renaissance de-corrosion fluid, sparingly applied with a paintbruch, and then brass brush scrubbed after a few minutes. and then repeated, in 2 inches portions.
It's working remarkably well in a short period of time, I only gave it a go for a bout an hour then wiped the rest and oiled it. There is bare metal showing but large portions of black pitted metal. It looks like some sort of evil fantasy blade now
I think I'll try letting the Renaissance de-corrosion blue fluid sit on longer. It evaporates quickly so maybe adding another later with a paint brush would help, and giving it 10 minutes to work.. I'll try again in a day or two and post pictures.