What about a nut splitter?
I f y' don't know what one is it's like a clamp that fits over the nut with a wedge shaped (or similar shaped) part that aproaches the flat of the nut at right angles. Y' wind it in with a a spanner or in this case maybe a socket with extension, keep winding until the wedge shape bit splits the nut.
OOps, just seen the last post, y've already done it!!