Hope you don't mind a question on the MG 1100.
I have to remove a rear hydro bag because it's leaking.
Strangely the manual doesn't tell me how.
Do I have to remove the fuel tank or the swing arm to get it out?

You don't need to remove the fuel tank or trailing arm to remove the displacer. (the trailing arms can be a bugger when trying to undo the sway bar and helper springs)
Jack up the rear of the car.
Depressurise the side of the car you are working on. (I use a tyre chuck with a bit of hose into a plastic bottle)
Undo the displacer hose joint under a rubber plug inside the boot.
Then remove the displacer past the trailing arm.

When you reassemble it make sure the strut is sitting correctly in the displacer as you pump it up.

Is the displacer leaking from where the hose joins the displacer? You can fix that. There is a few different ways. Take a look at the Austin America website.

You do not want to be fiddling with the sway bar and trailing arm.... trust me

There is a trick with the sway bar, you remove the bump stop so you can lift the arm higher and then you can undo the bolts through the sway bar with a socket. It all works if you hold your tongue the right way.

But it is only a few more bolts and a couple of hoses and the whole subframe is out!

I walked out one morning to the garage and there was hydro pee pee under the rear end
and the car sagged to the bump stops.
I'm assuming it's the connection to the bag.
It's jacked up, but I've got to buy a big enough open ender to undo the pipes.
Just couldn't see how the bags would come out.
It's a relief to know that's it probably easier then I've been thinking.

As it turns out it's not displacer. It appears the pipe has worn a hole on the inside of the bend where it passes over the subframe.
Probably more difficult to fix or replace than the join at the displacer.