System is Slackware 15.0, current kernel is a handbuilt version of 5.15.19 with the patch for a scrolling tty.

I was at a console and idly gave a command which had a lot of output. The system froze and I couldn't unfreeze it or go to another console. So I did ctrl-alt-del and this message came up:
Kernel BUG: unable to handle page fault for address ffff8acbc35d800

The system refused to reboot and I had to switch it off at the main. Afterwards I went through /var/log/messages but couldn't find that message or anything else that looked significant. So what happened and is it important?

can you reproduce (repeat) it? I guess it is the "scrolling tty patch", so I would try the same thing without that patch too.

Plenty of serious bugs fixed since 5.15.19, see slackware 15.0 changelog for details.
I'm on 5.15.39 that was released this morning or sometime last night.

The latest kernel in patches is 5.15.38. I think I'll install that one in a few days (new download period starts on the 19th, yippee!). In the mean time, over the next couple of days, I'll try

1) reproducing the error with 5.15.19scroll.
2) booting with 5.15.27-generic (Patrick's build, no patches) and see if I get the same thing.

I remember from way back that you had a download limit.
Here on mainland, we have some pubs with free wifi, so people with no internet at home often sit there and surf all day.
Don't know how it is in UK, maybe you could ask around, download a kernel in the pub without spending your monthly cap.

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Actually I've bought a 10 GB top-up as an emergency buffer, and there is no expiry limit on that, so I won't get throttled or cut off again as I was a few months ago. But the habit of being frugal with bandwidth has become ingrained. Put it down to a childhood in the 1940's.

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Kernel gets larger every year, so I understand it's taking more and more bandwidth to keep up to date, but;
Flat rates around here usually have around 1-2TB soft cap, upon which they are throttled (not disconnected).
Never reached it personally, but I do know some repeat offenders saying it's only throttled until next month.
Pretty sure only mobile/satellite providers still charge for data, cable has been flat-rate for a long time.
Public places such as libraries, I don't think they even have a data cap they just limit the bandwidth.

Take your laptop to M&S and use their wifi. It's free and no snags to start, no strings.
You can have a very nice coffee while you work.

Who said anything about laptops? I do have a laptop as it happens but it only has 1 GB of core and I regard it as a toy. I certainly wouldn't build software on it. My working machine, the one I run Slackware and LFS on, is a good old-fashioned tower with a screen, a proper plug-in mechanical keyboard and a real mouse complete with tail! None of this bluetooth stuff!

I'd recommend:
Used android with SDcard for downloading over free wifi.
Cheap chinese USB connected SDcard reader for your desktop.
Edit: might not need a reader if the android is in good enough shape to connect to your desktop.

Ha! Ha! What makes you think I have a smartphone? Nah, I'm too old to learn to use one now.

Well, being old and tired (starting to have mobility problems) I spend entirely too much time on the 'Net and often a good portion of the day is spent streaming old movies or TV shows. The Brits had/have some great TV shows. The local ISP has a 1TB limit before adding additional fees. I check the usage every time I pay the bill and have never exceeded 80 gigabytes. Might as well use the highest possible resolution, but I don't, as I find it distracting more than anything else, that is, it doesn't really "improve the experience."

That's what I thought when I got one, but it's not really smart, it's just linux with some java stuff patched on top.
Mine is in a really bad shape though, usb is wrecked and it can barely charge, but sdcard works great.

To get back to the point, what's the difference between a BUG and a panic? I might as well learn something from this.

BUG is the reason and panic is the result.