Getting There
My thanks to all for their constructive help - especially to UNIXMAN for suggesting lsof, which I'll be using when all is back to normal.
Progress to date is that I have rebuilt the OS disk from scratch, but I was unable to persuade the OS to let me rebuild the RAID. However, I managed to mount one of the RAID disks to recover the information - a full backup is running at the moment. When the backup is secure, I'll try to rebuild the RAID without reformatting both disks, in the hope that it will recover the mirror - any clues as to how to best go about this? At the moment one disk is Ext4 - I was obliged to reformat it after the initial OS rebuild, but the other still thinks it is part of a RAID - albeit I have managed to mount both disks as /tmp and /tmp1.
The current rig has a 3 GB partition mounted as /SWAP - would swapd be more efficient? (I think I see what you are getting at - might the swap file have grown to fill the data space? Answer is, I think, no - as I only have a fixed swap partition).
......."Weird Trip, Man" (Oora - Edgar Broughton Band, circa 1972).