Very many thanks, everyone, I really appreciate all the comments and observations. I've decided to get one, and the input from here has very definitely helped the decision making process. In case it's of any use to anyone else, I went about trying to decide what to buy as a replacement laptop having already thinned the options down to three, a Dell XPS13, that I'd have changed to running Linux (for a host of reasons, not least of which is that I'm just getting fed up with Microsoft), a Macbook Air, or a 12.9" iPad Pro plus a keyboard (probably the Logitech Folio).
The Dell/Linux option was already in third place, really, as, although I know the XPS13 is well supported by Linux, I've also experienced more than a few head scratching moments getting laptop hardware to work consistently and well with Linux over the years. Linux tends to fall over a little bit on some laptop specific stuff, like touchpad functionality and power management, in my experience.
The harder choice was between the iPad Pro and the Macbook Air. My wife's on her third iPad, and I've used hers from time to time over the years, so I'm reasonably familiar with what it is good at and what it's less good at. I've also had an Android tablet for a few years, although I don't use it much. What I have learned over the years is that I very much prefer a proper keyboard (I'm typing this on a big clicky Cherry one) and I find it much harder to type quickly on a touch screen. For this reason (and a few others) I don't much like using my iPhone for anything that needs typing. The biggest plus point for the iPad Pro was definitely the screen, probably the very best screen of this size I've seen. Clear, very bright, high contrast and fast. The sound quality is damned good, too. The biggest downside with the iPad Pro was that with any keyboard attached it doesn't really work well balanced on your knees at all. I spend a fair bit of time using my existing laptop like this. The other negative is that the iPad Pro wouldn't work with our network colour laser printer, and iOS still has some file sharing issues that make is less than easy to work with a home file server.
The key reasons for opting for the Macbook Air were the ergonomics, the keyboard feels good, the hand rest area is nice and generous, the screen is very good (although not as good as the iPad) and the build quality feels very solid (something that seems to be borne out by the experiences above). There were also a few MacOS features that swung things a bit in its favour, like the ability to send texts with it linked to my iPhone (another keyboard typing plus for me) and the seamless way that Apple manage to make a bunch of different devices all seamlessly sync together. My wife loves this feature, and has her iPad, iPhone and iMac all seamlessly connected to each other.
I had wanted to switch everything to Linux, but having tried to get my wife to use Linux (I had a PC set up for her running Mint for about a year) she didn't really get on with it at all, and found the learning curve in getting things to work well with Linux too steep. Mint is very good, but there are still too many things that just aren't anywhere near as polished as even Windows, and these can cause a challenge for someone that doesn't want to delve into the nuts and bolts of the system from time to time. I'll still be running Linux on a few boxes around the house, like the file server, a Pi Hole ad blocker and TeslaMate for gathering data from the car API, but I think that we may end up with most of the day to day stuff being Apple in the end.
Yes, I know that I'm buying into a closed ecosystem, but then the same is pretty true for Microsoft and Google now, except for the hardware, and there don't seem to be many flaws with Apple hardware, barring the relatively high price. It's certainly very much a personal thing, but if I had to rank these three companies in terms of trustworthiness, bottom (by a long way) would be Google, Microsoft would be significantly more trustworthy, but for all their flaws Apple are probably the most trustworthy (which isn't to say they are always to be trusted, it's a relative thing, IMHO).