Took the door apart again today while I wait for the eBay seller for the other lock to get back to me.
I think I know why the handle bit wasn't tightening back up now...
The ring must have seized solid and then popped off when I tried loosening it. Luckily I had the spare that I'd taken apart and cleaned and greased to pop straight back in.
Ruined the perfect butyl tape job I did when I sorted the seals, but it'll work fine and it'll be hidden under the door card anyway.
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