After a number of weeks, things appeared to have worked well.
This stem snapped easily and the fibrous outer peeled away. By my index finger there are some quite fluffy fibres. All good.
After separating what fibres I could from a number of stems, it became clear that the first one I picked up wasn't representative. Some peeled like rhubarb, the fibres in a thick ribbon, like blades of grass (I assume the retting hadn't worked well in those cases) and other stems had nothing, or a black mush that wasn't fibrous. Maybe in those cases the fibres had been eaten / rotted away, or the stems weren't good in the first place because I harvested late.
I noticed that the best ones felt slimy. This is backed up by the fact that when I left all the fibres I'd collected in water, they all turned slimy. I take this to be the bacterial / fungal action that we're after with the retting (if anyone reading this knows about these things and can confirm or deny this, please comment).
After soaking for a bit longer, I did have a fibrous material. But there were too many thicker fibres in there (maybe not separated / broken down) and trying to draft and twist them between my fingers produced what looked and felt like twisted grass - not what we were after.
When dry, they felt brittle and coarse. I decided to cheat on the 'apocalypse' idea and see what happened with carders (hoping the fibres would separate).
There are a few fibres on the carders. But a lot has just broken and flaked. I think it's fair to say that this hasn't been an unmitigated success.
I'm going to try again in the autumn, harvesting earlier and trying the quicker retting method in a trough or the bath.