After much faffing and I've established that the data is probably all okay, but the partition table on that drive was AWOL. The brilliant and free Partition Assistant has just sorted that, so now I just need to wait for FlexRAID to do lots of complicated maths (15 hours!) and then all should be well. Phew.
But wait, there's more.
In the process of this I discovered that my server hasn't successfully completed it's scheduled back up or parity jobs since some time in July, which is rather sobering. I have just dodged an almighty bullet.
Kids. Make sure that you make regular backups and that you can actually restore from them. 