They were planted correctly in their supposedly favourite clay soil,
but this year the Evereste has one apple, and the White Star about 20.
. . .
They came from a reputable East of Scotland grower, but I can't help
feeling that they must have been forced to have been so laden with
apples in a 10-inch pot.

Don't know a whole lot about crabapples in particular, but often the
mere putting of a plant in a small pot will encourage flowering (I
believe the evolutionary origin of this trait is that a plant which is
stressed in various ways tries to reproduce, in case it doesn't make
it). No doubt there are other ways that the nursery could have forced
the flowers. Since plants which are in flower sell for more than ones
which aren't, it isn't especially shocking, nor especially worrysome
in terms of your long-term success with the plant.

My rule of thumb with woody plants is that I don't expect
flowers/fruit for at least the first year (longer for some plants).

If it goes another year or several without flowers, then I'd start
wondering about fertilization (e.g. too much nitrogen), not enough
sun, etc. But the fact that you didn't get a bloom the first year
they were in your soil doesn't strike me as unusual or problematic.