[alasdairc]

Exactly like the instructions on github

avr-gcc -Os -Wall -DF_CPU=16000000UL -mmcu=atmega328p -c -o usbto433.o usbto433.c -lm I.
avr-gcc -mmcu=atmega328p usbto433.o -o usbto433
avr-objcopy -O ihex -R .eeprom usbto433 usbto433.hex
avrdude -b 57600 -p atmega328p -c arduino -P COM5 -U flash:w:usbto433.hex

No errors reported. It looked exactly like every other flash I've done over the years.