Thank you so much for taking the time to help 
So with “centre cropping”, this is what I would like to have on the final video
But this is what I get on export (extra black bars cutting off the image)
720p resizing (centre cropping)
With the 200px on top and bottom it cuts too much of the image:
…and curiously I get the same video result as above 
720p resizing (centre cropping)
Here is my .mlt file:
720p resizing.mlt (3.9 KB)
I hope it helps…!