Minor mystery resolved
A couple of months ago, I made my usual apple and sloe jelly.
It took me three bloody goes to get a set: I ended up sticking pectin in it (FFS!), and that eventually did it.
Two days ago, my medlars were finally bletted:

(Crafty Day folks, I suspect that if I'd hunted around then I could have found a bletted one for you to try. You'll have to come back around the same time next year.)
So yesterday I boiled up the medlar jelly and this morning... no set.
Some experimentation with a pan of boiling water revealed that my jam thermometer is reading a degree too high.
All is now explained. Although I fear that, in my attempts to be over-cautious, extracting the re-boiled medlar jelly from the jar may need a sharp knife rather than a tea spoon. Especially as I've just seen in my diary that I didn't bother with pectin last time.
It took me three bloody goes to get a set: I ended up sticking pectin in it (FFS!), and that eventually did it.
Two days ago, my medlars were finally bletted:

(Crafty Day folks, I suspect that if I'd hunted around then I could have found a bletted one for you to try. You'll have to come back around the same time next year.)
So yesterday I boiled up the medlar jelly and this morning... no set.
Some experimentation with a pan of boiling water revealed that my jam thermometer is reading a degree too high.
All is now explained. Although I fear that, in my attempts to be over-cautious, extracting the re-boiled medlar jelly from the jar may need a sharp knife rather than a tea spoon. Especially as I've just seen in my diary that I didn't bother with pectin last time.

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At first I thought you were making up words, until I googled.
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rottedbletted.