I had a look in /boot/config.txt, but there was nothing of use there. I did come across something else though;

https://github.com/tasanakorn/rpi-fbcp

This utility will copy all the data from my primary framebuffer (fb0 / HDMI) to my secondary framebuffer (fb1/ USB). I've set it up to auto-run on start-up and it kind of works now. My next problem is that the refresh rate is only around 1 fps!

@Lyle_JP , Thanks for the reply!
I'll have to wait until tonight to check it out.

The TV is in fact 1360x768. But I believe I tried group=2, mode=39 last night , after checking some of the resolution modes on Pi's website. I don't believe I tried as high as 81 , though. I'll check it out this evening.

@bfenty The script runs automatically on boot, so you just reboot the normal way.