I'm getting lots of queries in another place today.
What would you fill this gap with?
I think it is a newish house drying out.
So my comment would be Decorators' Caulk, or perhaps silicon if it is still moving.
I have fired off a couple of further questions.
Update: Answers
"The home is coming up to 3 years' old. I believe it's just a result of the house drying out/settling. The 'crack' hasn't changed much, if at all, in the past year or so.Unfortunately Bellway refused to fix it because the crack wasn't big enough."
Cheers
Ferdinand