Question Author
Good news Chris! I had another go at changing the resolution on the display, settings, tab like you first suggested and what was happening was that when I first went in, the resolution was on max - right up at the right hand side. I nudged the lever to the left and got bigger print, objects etc, but a reduced screen area with a big black border, like I explained. So I tried again, nudged it a bit more to the left this time, with similar results, and so I thought it wasn't working.
What I've discovered today is that the screen area only normalizes back to fill the whole screen when you move the lever to below the halfway point. So it's all looking much bigger now, and filling the whole screen.
Thanks a lot for your help. Cheers.