@ian57 Wow - that was the problem after all this. Thank you 
In my defence, it didn't appear that I needed these settings as the Recalbox front end looks perfectly fine - pixel perfect, no glitchy pixels due to wrong resolution, the full screen is used - and indeed, the other emulators have their own black bars on the side and are already aspect correct. It was only Dosbox with the problem. In fact I only went and applied these settings just to say I had, I didn't expect it to make a difference...
