I would be very surprised if a dead blow hammer wont get them going. One hand over the nut, holding the spanner in place and a few sharp taps with the hammer. Easy to forget that the off side is a left hand thread but sure you are well aware of that.
Got it sorted thanks (see above) but it wasn't a few sharp taps - more like a flurry of blows for a sustained period of time! (after heating first). Used to come of easily with my long-handled spanner.
