Thanks, but that wasn't really helpful.
The problem was due to my second monitor being smaller than 790 px (1360x768) so even though I was playing the game on my HD monitor which can easily accommodate 790 pixels, the game would not recognise this. Disconnecting my second monitor allowed me to set the resolution to 100%.
You didn't mention that you had a second monitor.
Glad you can now play at 100% on the one.