Perhaps the soil is just a little too dry for a lot of worm activity. Have you had any frosts? That will make the worms go deeper into the soil to stay warm.
Also it depends what type of leaf it is. Beech leaves take 3 yrs for them to rot in my garden.

I think dryness may be more of a problem than anything else. Even after the infrequent rain we seem to get here, the soil is still dry below the surface. Any water which does get in is soon extracted by the tree roots.
The leaves were probably a mixture of Beech and Birch, but even so, after 18 months underground I would have expected them to have at least begun to change.
They were buried wet, by the way.

That's very mysterious and disappointing. Perhaps you might have some success if you bought some brandling worms (is that you you spell it?) I know some folk buy them to get a compost heap going. Or perhaps getting some used mushroom compost - I remember my Mother getting some once as a soil improver but can't remember anything about worms...

Owdboggy, I think I have a similar issue. Hardly a worm in sight when I dig even though I add garden compost from time to time. Buried leaves (mainly red oak) don't rot down even as well as they do in my leaf mould black bin bags. The soil is clay. Hundreds of worms in my compost bins, but very few earthworms in my borders.

the compost bins are usually inhabited by a different worm species [brandlings] to the ones that live in the soil. It is also wetter and warmer in the compost bins.

And another good idea bites the dust...

If you can get hold of earthworm castings at the local garden center, applying those will attract worms from the surrounding area. Those contain kinds of soil bacteria that eathworms find very tasty. Compost tea will also help, but is a more complex project.

Never seen them on sale anywhere in the Garden Centres/Nurseries we go to.

Owdboggy, I would expect them to be less common in the UK than they are here. I imagine that most parts of the UK have enough wild earthworms to make their castings part of the normal soil composition. I was hoping that they were at least occasionally available from knowledgable garden centers. Maybe the keyword is "knowledgable", as is too often true here!
Ah, well! A good 48 hour compost tea may be the way to go, then.