Thanks for replying.
I’ve reverted to just using regular size thumbnails (400*157), and will go with slightly different design layout.
When I tried using 400*650 and the css they come out stretched (as you point out).
But if I right click on the image it says the actual size is 172*300, which is not what the actual featured image size is. The images I was attaching to the pages were definitely 400*650.
So I think WordPress is somehow getting involved “auto changing” the size – is it possible that the media size settings in the admin are effecting things?
Thanks
Malcolm