The house was built at the same time but it's obvious the main 3 storey building and the single storey "extension" are separate structures, they share a join along 1 aspect which is where the hairline crack manifests.
Personally I think the heavier building has settled more/quicker than the smaller building (clay soil) which has pulled the wall down and opened the crack as force has been applied to the top of the wall, throwing it out of square.
I would hope NHBC are sending qualified people to assess issues, if not I'm wasting my time even asking.
I'll continue to monitor the crack, the report is on record and there's over 5 years left on NHBC warranty.
I think the jist of what he was saying is that the main 3 storey building moves at a different rate than the smaller single storey extension leading to forces between the two.
Makes sense as no signs of damage on the main building and other similar properties on the estate have a similar hairline crack in the ceiling plaster where the two buildings meet.
He did say a wall tie may have corroded or failed internally leading to the smaller wall kicking out or getting dragged in.
Yep, written report stating...
"The damage is consistent with shrinkage/thermal movement"
Didn't specify his qualifications.
I've repointed and will monitor for further movement.
Update:
So NHBC completed their inspection.
No evidence of subsidence, cracking due to shrinkage/thermal movement.
Repoint, reseal, keep an eye was the advice.
Guy seemed to know his stuff, and was an ex subsidence assessor.
Happy with that, good to know nothing sinister going on.