A little trip to the Internet informed me that Error 24 meant it couldn't drain. Given that the bottom of it was full of water, I had already deduced this. Fortunately, there were plenty of videos offering instructions on how to fix an Error 24.
Take out the trays the plates go on, OK, that's easy enough.
Remove the filter that stops bits getting into the gubbins. OK, I could do that, the water wasn't deep.
Use your industrial vacuum machine to take out all the water. Hmm. That would be the industrial vacuum machine I don't possess, then. I had to sponge it out.
Remove the weird-shaped piece of plastic shielding the impeller. I was worried I might break it, but it soon yielded to my brute strength and freed itself from its housing.
Feel around the impeller (which is basically a fan that works on water) and remove all obstructions. Oh. There were no obstructions.
If there are no obstructions, use a drain-unblocker. Right, well I did that, using half a bottle of the stuff that my wife kept under the sink ("Not that sink, the other sink!". It was that sink.)
Nine hours later, reassemble the machinery and try it again. OK, I did that. I gave the filter (which is like a highly perforated tin can) a nice clean, because it had a lot of fat on it, but not enough to impede the water flow.
Try the dishwasher again. I did it with a prewash, because I only needed to see if the water drained away.
Error 24 again.
Back to the Internet! If clearing the pipe doesn't work, you need to move the unit off the wall. Start by unscrewing interior panels to remove the motor, then switch off all power. Disconnect these wires first. I don't know what came next, but the impression I gained was that a bomb would go off if I didn't do it right.
Well, I wasn't going to get into dismantling a motor. I resigned myself to buying a new dishwasher.
Overnight, as I slept, it occurred to me that the amount of water at the bottom of the machine following the prewash test was much less than it would have used. It was draining, but something was stopping it.
Next morning, I got up and checked the filter. Something I had previously believed was a part of it turned out to be no such thing. This is what it looked like:
I don't know what it came off, but it must have fallen into the filter and prevented it from turning properly.
I took it out, my wife cleaned up the filter (unimpressed with my earlier effort), then we tested it on a prewash again.
It worked!
We filled it up during the day, and then ran it.
It did a prewash, because it remembered the setting, but it finished. I persuaded it to switch to a regular wash and set it off again.
Error 24.
Overnight, I once again worked on what had happened. It had definitely drained away the prewash, so the pipes weren't blocked. It then occurred to me that perhaps I hadn't reassembled it properly. I seemed to recall having had to turn the filter to get it out, but I hadn't turned it to get it in.
Next morning, just as my wife was about to spend half a day looking at dishwashers on the Internet, I tried twisting in the filter.
Something clicked, so I retreated and set the dishwasher going again.
It completed! No Error 24!
Our dishwasher in now serviceable again. I've no idea what that piece of triangular plastic came off, but I dare say I'll find out eventually when whatever it is falls apart.
In other news: "Openreach: We're sorry we couldn't get you connected today. The job is a bit more complex than usual. We are going to get it sorted though and promise Full Fibre's worth the wait. We'll be back in touch in the next few days.". They provided a link to a web site explaining what the possible problems might be: underground works; tree cutting; scaffolding may be required; road closures; gully services; specialist engineer is required to complete the work; permissions may be required. Strangely "bloke with a ladder was busy" isn't there.