I was flying on a job with 2 other pilots who were also running the Inspire 2 and when I got the rushes into the edit, I noticed that they were all suffering horizon drift.
I am now having a battle with DJI NL about this so I'm keen to know how may others have this issue.

Footage sped up to show issue more clearly:

Was this at Dartford crossing?

No it was at Southampton

Woowww! Nice shots!

Could have been better without a wonky horizon though

I think all the Inspire 2s do this.  The reason why others look so stable is because they are using software to stabilize it up in post.   You are supposed to shoot in 4k then downscale it to 1080p where the computer would make it look super steady.  Just because you bought the latest best DJI all-in-one drone today doesn't mean it has all the problems worked out.  It's like expecting Skydio 2 activetracking quality back when the best drone was a Mavic Pro.  It's like expecting Toy Story movie graphics back in N64.  The M2 is such an amazing design.  It would be nice if they just made it 3x bigger to accommodate a bigger sensor.

That seems about normal for almost any dji drone. They all got a drift in the horizon.
You can fix some of it preflight by calibrating the gimbal on a real level surface (use a bubble level to check).
Any declination can be fixed then manualy in the settings. But be aware that these settings can be lost over time inside the app.

You can also assign roll on your remote so you can fix it while in the air.

Or just fix it in post like we all do ;)