Thanks Dan-O. Yes you were right, even with the lost GUI the BIOS version of RAID identified the drive and the rebuild started.
Unfortunately, the initial degradation had caused corruption to the OS install (specifically Services.exe) so we had to do a restore from tape to get the OS to a point where we could interact with it for more than a couple of minutes post boot up. It was slowing down to five minute response times whilst Services.exe could not run. Unfortunately I diagnosed the slowness as a rebuilding RAID array and put up with it for a day before rebooting and seeing that the RAID was rebuilt (in BIOS) and working out that there was some other problem.
So it goes. Thanks for your help.