I have attempted a fettle of the shower, it didn't work.
I guess a new cartridge and strainers are needed, I disassembled everything, gave it a good clean, a good staring at*, then put it all back together. Then I took it all apart again, gave it another good staring at, then put it back together correctly.
*An essential part of the fettling process, I find, is staring out the object you are fettling, just in case it had any ideas about not going back how it should.