Many thanks, the Itch version works perfectly.
I tried replacing the Steam one with the Itch but it ended with the same result.
Just out of curiosity I decided to launch the original Steam version directly from the Steam folder and that actually works OK as well. Obviously the issue must just be a facet of launching from within Steam itself.
I tend to buy most of my games from Steam as their Linux support is getting to be quite amazing. It would be a great shame if the launch issue is not solvable as I imagine many people would just ignore and delete a game that won't launch. As I myself would have done if it hadn't been such an interesting looking game and, equally, thanks to your outstanding customer service.