Am I missing a knack to removing the locks or is it just PlusGas and brute force?
If every effort with a spanner and screwdriver fails, you will have to cut one flat back to the thread with a Dremel or similar small grinder. Once you have cut through completely, all the pressure is relieved on the thread and it should undo easily. The problem is corroded metal is bigger than uncorroded metal, so it stops it turning purely due to size.
If you need a new large nut, I have plenty spare as I changed 14 door locks for new.
Thanks for the tip - will have a good go with my assistant, and if that fails, get the Dremel out.