See, my client
So I picked up a spare disk from the lab and went to Birmingham. I moved one of the disks into a different slot in the cage and it ceased to be missing. Whatever I did in that slot failed, so I think that the corresponding connector at the back of the cage is broken.
Once the disk was talking to the controller again, I tried to boot.
The array configuration utility now recognised that the volume existed, but claimed its status as "uninitialised" -- but despite this NT booted normally. This made me nervous.
I checked the server out. About 700Mb used on a 4Gb C: partition, about 17Gb used on a 64Gb D: partition.
I cloned the contents of the volume onto my spare-disk-from-the-lab, tore down the RAID-5 array and instead made a RAID-1 mirrorred set with a spare disk left over, to comply with the client's revised server-config guidelines, and cloned the contents back again.
Everything started normally, so it's off to the hotel overnight for me, and back in the morning to confirm that the damn thing still works when the girls log in.
Dinner on expenses now.