Hi Peg,
Thanks for your reply. I did do a thorough search on the forum, but the only thing I found was that you could set the backdoor key as 0xFF for all eight bytes on the HCS08 parts.
I think my real issue is that one datasheet claims that the GT16A is different to the old GT16 in that you can store the key in FLASH and write it to the backdoor key register from there, whereas the device datasheet contradicts it.
I don't want to put anyone else out of their way to test this, I was just wondering if anyone had done this already and remembered which one is telling the truth.