Mine didn't have an extension. It was just called VirtualCache. Lived in the Products folder (i.e. outside the PUBLIC_TEST_SERVER_64 folder). I tried moving it somewhere rather than delting it, in case it was important, but the installer just recreated it and the game updated properly.
Subsequent failures have been dealt with by deleting *.part and *.progress only.
Yeah it did, you just couldn't see it coz you haven't got "show extensions for known file types" turned on. All the extensions are still there but windows hides them by default, but I started using PC's under DOS 3.0 and I miss them when I can't see them.

Where are the .part and .progress files? From the names I'd guess they're download manager files for partly downloaded updates.
Cmdr Marc_Hicks has very kindly emailed me not only the data.ovl I was having problems with but two more than he'd had problems with too. I put the first one in and was able to get the download to continue then had issues with the second one he sent me. Have since replaced that with HIS version and the download's continuing just fine again now... and I'm just waiting for the third one he sent to cause me problems. *SURPRISE* It just popped up while I was typing that, and it WAS the same one he'd had problems with. I've just restarted the download and it's working fine again as expected.
The interesting thing about this is it proves that it's not just some random isp or router induced error. If we've BOTH had problems with the SAME three files then it HAS to be something at FDev's end coz that's the ONLY common element. Download's about half finished, Will watch with interest to see how the rest of it goes.