Thanks John.
The .dbx file named Deleted Items on the hard drive is some 370,000KB. (It's roughly in correct proportion to the Sent and other Folder files, so I'd assumed that the relevant number of individual messages are/were still in there!) The question is, are these messages just misplaced and inaccessible due to a quirk within OE, or have they been "first-stage" deleted only? The File, Import procedure won't take me beyond a "files already in use" panel while OE is open of course....
The other problem which has occurred is that suddenly, at almost the same time, a hefty extra near-one-GB of my 3 GB hard drive space was used up over one session (normally FAT32 fluctuates by a meagre amount, but this is massive) - despite the fact that I've had no sudden download activity or anything that could explain it.
I'm wary of defragging in case I lose the dbx contents in doing so.
Where next?