simplex:
avrdude in conjunction with Arduino Uno flashed with ArduinoISP should be able to change the fuses after a bootloader or some other hex file has been uploaded to the target Attiny.
Yup, thanks for that 
I totally forgot about using avrdude as a standalone.
This worked:
avrdude -CE:~Path~Removed~For~Security~\arduino-1.0\hardware/tools/avr/etc/avrdude.conf -v -v -v -v -pattiny84
-cstk500v1 -P\.\COM4 -b19200 -U hfuse:w:0x57:m
Rather pleasingly the blink sketch I uploaded to check with continued blinking away after avrdude finished
Edit:
Just checked and the bootloader is still functioning properly. I have to remember to do a power cycle just before the program starts to download to get back into the bootloader, but that was to be expected. Bye bye autoreset