Remove the big octagonal nut. The correct tool here is a 1-61/64" 8-sided socket. The one shown here is sold through major British parts suppliers. The back end of this socket has a square hole for a 3/4" drive adapter. Be prepared to handle at least 150 lb-ft of torque to loosen the nut, although many of these will be much less tight when you get there. The nut on the right side hub has right handed threads, but the nut on the left side hub has left handed threads. The one on the left side can be very frustrating if you don't remember to turn it clockwise to unscrew it.

To confuse things even more, the later MGA MK-II and MGB models with banjo axels MAY have right handed threads on the left side hub nut as well. Here the emphasis is on the word "may", as they could be either way. Usually there is enough of the male thread exposed beyond the front surface of the nut that you can follow the thread for about one turn to determine which way it leads.

Note also the large hole through the top of the big socket. If you don't have a 3/4" square drive socket handle or adapter, you can stick a lug wrench or a round tire iron through this hole to turn the socket. Alternatively, you may be able to find a 4-wheel-drive socket of the appropriate size at a local auto parts store.

More on rear hub nut wrenches. - - More on rear hub nut torque.