[jools-elliott]

Interesting.

I looked at this back in Lightroom. If I run the levelling tool across the "horizon" then there is a massive shift. However, it then makes all the buildings lop sided.

If I keep it as it is and look at the top of the chateau, it's pretty much bang on!

Would hazard a guess that lens distortion coupled with the merging process has introduced some irregularities.

*Edit.

Just played with one of the regular images I took. The horizon is straight. It is an illusion caused by the camera angle and the camera being pointed slightly upwards. Some tilt and shifting is all that is required in this case.