Grunff wrote:

Background - our house was built in the 30s, has a square footprint
around 8x8m, and has no foundations to speak of. It's made of stone +
lime mortar, and the walls are between 50 and 60cm thick.

snip original post

The structural engineer has just been and gone. He appeared to
be very knowledgeable, and had lots of qualifications.

In short, he believes the wall has moved both downwards and
bowed outwards. Partly due to settlement of the cut and fill,
and partly due to the rear porch having no discernable foundations.

Top marks to mark for coming to almost exactly the same
conclusions without even seeing the place!!

He (the engineer) said that there's no need to take any action,
but that there may be some future movement, accompanied by
cracks to the interior plasterwork.

He suggested that putting in a tie or two, tieing the back wall
to the floor joist(s) may alleviate the cracking. Which is oddly
where my question started out...

So there. Nothing to worry about. Thanks to everyone who gave
sensible, reasoned advice.

--
Grunff