Hi purple_lily.
I followed PeterVH's instructions in order to download ArduinoISP updated version. Then you have to back up your older version in arduino/examples folder and put the new one inside. After that I tried to upload it to my ArduinoDUE and everything worked fine! In the meanwhile I've been a little busy and I was also waiting for some attiny from farnell. I think I'll try to make it work in the next days.