Replaced the files with those from debug.zip and all buttons are now loading and displaying correctly.
I'll play around with it a bit more, but that part would now appear to be fixed - many thanks!!
I still have the problem that if I plug anything in to my laptop while running dmxcontrol, such as an sd card or usb device, the stream deck freezes.
To get it to work again, I have to unplug it, and reconnect. When plugged back in, I then have to press all the buttons to get them to display again.
Any ideas on anything that I could try?