Finally. All up and running and happy.
I added a 4th disk that is a different capacity, then used acp_commander to force the format. As I have a screen and keyboard attached, I did try running the command locally, but it asked for the root password.
Thank you so much for all the help.
I've gone from feeling stupid, to feeling relieved that I hadn't missed anything obvious and it just feels like this device was conspiring to stop me.
Now I need to find some subtle way to put a big note on it so I don't repeat my mistake.
Just need to wait for the RAID sync to complete and I can power it off, pull the USB stick and put all the screws back into the HDD's
Thanks again
Chris