Belated thanks for all the new replies.

The current situation is that the disturbing noises have almost entirely
stopped: I still hear the very occasional single noise, more a creak than a
crack and nothing like as loud as previously.

I'm inclined to agree with everyone who suggested a greater than usual
degree of shrinkage in the timbers, possibly specifically the floor/ceiling
construction. What very minor cracks are evident in the walls have been
there since shortly after the job was finished and have never increased (and
one day I really must fill them) so after a minor initial semi-panic I'm
inclined to think that nothing serious has happened or is still happening.
One change that is noticeable is that the floor of the new loft room is
definitely noisier and has very slightly more give than previously, but not
enough (I tell myself) to be alarming.

Tomorrow, when the entire structure collapses, I might have to eat my
words...