When I turn the device on, I get nothing but seemingly random numbers scrolling quickly across the top of the screen, and the device is unresponsive.
That's new to me. Please post photos of your board without LCD.
M328 is inserted correctly?
Perhaps something to do with the rotary encoder. Try disconnecting it.
The M328 (ATMEGA328P-PU) is connected correctly. I have re-programmed 6-7 times now between the r523 firmware mentioned earlier (which works) and the latest one from the repository (which I have the trouble with).
I have now tried compiling the latest firmware myself both with and without the "DWITH_ROTARY_SWITCH" and get the same result as the precompiled from the repository (scrolling numbers along top of screen).
I tried the 1.12k_r523 firmware again to check: The rotary encoder works with the 1.12k_r523 firmware (you can see the r523 firmware running in the attached picture).
Photos are attached of front and back of board, as well as another one where the scrolling numbers (that I am getting with the latest firmware) are easier to see.
Have you programmed the EEPROM too?
If you mean the .eep file to 'data memory', then yes. I am also setting the fuses as per the attached picture (which I sourced from elsewhere in the thread).
I guess unless there are other suggestions then tomorrow I will try removing the rotary encoder completely, and programming using avrdude instead of the TL866A - but I'm not confident either will make a difference...
Also - how do I check which revision/version number is the latest in the repository?
Thanks for your ideas!