Don't fanny about with the 8mm nut on number four, just notch out the cable terminal with a pair of side cutters so it slides under the nut, you then screw the new plug in with the 8mm nut allready in place.
That design of ratcheting ring spanner aren't brilliant, when you put a lot of weight on them the individual flat pieces of steel that they are made from bend and they come apart. If you do want ratcheting ring spanners you want the type where the body of the spanner is forged from one piece of steel.
As has been mentioned, the offset ring spanners are needed, that ratchet spanner simply won't go onto the difficult one on my '97 Xantia.