I'm answering my own question, the hub does have to be removed to replace this plate. The aperture in the plate isn't big enough to manipulate it over the hub. The plate is held on by 5 screws with 7mm hex heads. Amazingly, they all came out fairly easily. Clear away any grime away from around the heads, give them a squirt of penetrating oil, use a 7mm 6-point socket, and tap it lightly with a hammer to make sure it's over the screw head as far as it can go. With a slim drive extension that can pass through the holes in the hub, undo the screws with a socket wrench.
I can get a 14mm socket onto the heads of the 4 bolts that hold the hub on, I should be able to get a wrench onto them without having to do any more dismantling other than remove the shock absorber which is in the way.
Does anyone know what the correct torque is for those hub bolts?
Update, found it on another post, 68.6 to 88.3Nm