Thanks for the response Peejay.
Now that I've actually got my hands on the unit, I can get a bit more info about it. Its a Pioneer Carrozzeria AVIC-HRZ900. The entire system is in Japanese, yet the buttons on the front are all in English. There is an image of the unit here. I've had a quick search around but can't find anything much in English.
I've gone as far as Google translating photos I took of each menu option, but I fear language is not there. Just seems a bit weird that the buttons on the front are 100% english!
I wonder if Pioneer do a similar disk to the one you described above?