The motor back plate can now be rotated if you tap on it with a soft hammer, but there is no obvious way to remove it - no gap to prise it off.
So I took some measurements and made up a simple two leg puller out of some 10mm ally plate. It hooks under the bars that support the central bearing.
Using the puller, the back plate comes off quite easily. Make sure it comes off straight.
I thought that there might be a big O ring retaining the back plate, but it seems that it just a light interference fit.
Here you can see the chunky steel magnet backing plate and the neo magnets.
As soon as you get the back off, you should mark the position of the magent plate, in case the armature spins round. Make sure that when you re-assemble the motor, it is in the same position relative to the case.