Because I cant leave stuff alone i kept going!
It seems that you need to switch off the "RAID Array Error Detection Response?" so that the unit doesn't try and switch off at the first sign of trouble.
I also switched on email notifications so I could keep track of progress.
I then dropped a drive from the raid0, which caused it to error.
Removed the array
reformatted both disks as stand alones (I first tried just one disk and then mirroring the blank one to the unformatted but it errored and wouldn't do anything)
Then I was able to finally build the RAID 1.
It seems just deleting the RAID 0, then trying to build a RAID 1 just tips it over the edge!
good grief, it took a while but i got there.
Most importantly I ignored the suggestion to "restart the drive" at any point, if i did this I would need to reinstall the firmware.