Turns out the worms weren't dying because they were being overfed, or because the pH had gone awry, or because they were too dry.
They were soggy, waterlogged and anaerobic. I've been giving each batch of poops a generous squirting from the spray bottle because last summer the worms stopped eating when the set-up got too dry. Turns out that probably hasn't been strictly necessary *for quite some time*. The soil, i.e. the cat poops transformed into worm poops, was not just cold but dense, solid and, as mentioned, obviously anaerobic.
...how do I know it was cold and dense? I donned two pairs of plastic gloves and dug in.
Let me just take a moment here to say OH GODS YUCK YUCK YUCK AAARGGHHH EW EW EW EEWWWWWWWWWW!!!
There.
I dug out the wettest bits of the mess and pulled and picked out as many dead worms as I could see. There were lots. Then I added in some lightly dampened coir worm bedding I had left over from starting the smaller wormery (I do have two, one wasn't quite enough for three cats, even in the summer) as well as a fair bit of carefully hand shredded newspaper, and tried to mix it all in as evenly as possible. In the process I came across a handful or two's worth of live worms, so all is not lost.
The survivors were mostly mature ones, too, so now that they have better conditions with a good supply of calcium, it's only a question of time before there'll be lots of slick worm sex (it always feels like it should feel impolite to open the lid to find a tangle of worms, fused together and busy exchanging gametes) followed by pitter patter of lots of little worm feet. As it were.
Since the population has plummeted for now, I'll need to treat it as as a new start and build up the feeding slowly again.
What I learned:
I'll want to get a little plastic shovel or fork for gently turning the vermicomposts occasionally, to make sure the waterlogging won't happen again. Including more (as in any!) shredded newspaper or similar occasionally would probably be good, too.
And next time the wormery stops making its busy moist slithering noises I'll check it *straight away*, not in a couple of days. Because most of the dead worms looked pretty recent casualties, and there were whole groups just dead in their burrows in the soil, so it seems the conditions must've flipped from ok to lethal pretty quickly.
On hindsight I probably should also have made sure the bottom drainage hole is draining properly, but, well, I was a wee bit preoccupied with not thinking too hard about what I was at. Peculiarly though, it didn't smell too bad, just the slight whiff of methane of the anaerobic bacteria munching on dead worm. Blessings, see me count them.
I dumped the anaerobic gick (a highly technical term) with the dead worms and most of the undigested poops in the garden and will dig it a little trench tomorrow, once I've daylight. No, I won't be burying it where I'll be growing edibles - not that the free ranging cats of the neighbourhood have that qualm. And, really, if we're going to get toxoplasmosis from these cats, we'll have it already.
Unfortunately I wasn't really thinking about immortalising the... dead... worms... Er. Anyway no pictures of the disaster, had other things on my mind at the time, imagine that. Here's what they have now, though. The sheet of newspaper gives the little helpers somewhere to escape if things are still too wet. The stuff that looks like sawdust is bokashi bran that came with the wormery; it in theory has lots of micro-organisms that will help getting decomposition started again.~~
This was a lot bigger a job than I thought it was going to be, but Lessons have been Learned.
