If this is a recent occurrence, might it be related to a change of habit on a walk, eg too much or not enough water or food? Inadequate water intake can do strange things, and I find in winter I tend to drink less, sometimes too much less.

Or perhaps a change in gait?

I find that after (not during) a Hundred, my ankles swell up and remain swollen for 5 days or so. No pain, just the swelling.

found this
https://www.hiking-for-her.com/swollen-feet-after-hiking.html

Do your ankles swell at the same time?

Thanks for the suggestions, Raymond. General health is fine - when I had my current pair of boots fitted, the chap in the shop assured me that some swelling was normal, but I doubt that means a difference of two sizes between one's walking and other footwear. Will try taking my boots off, but I don't think it can be heat that's causing it, since it happened on the Winter Tanners when temperatures, while not freezing, weren't exactly tropical.

I recommend a health check up unless you have tried this

If you know your health is good, take your boots off every couple of hours and cool them

I've had difficulty getting my boots off after a couple of recent walks (and when I've been walking again the next day, getting them on again). The problem first occurred a couple of years ago - coming on rather suddenly, when my boots started pinching 40 miles or so into a 50 - so I tried going up a shoe size, which solved the problem until it recently started recurring. I'd rather not have to go up another size, since it would mean consigning to the bin a decent pair of boots which should still have plenty of miles left in them - does anyone know any alternative solution please?