Is what appears in the top left a scaled down version of the scene, and is the other three quarters ragged triangles?

Yes, that is a good description of what I'm seeing on screen. It seems curious that I have this problem with Unfinished Business, but not with the original game.

Hmmm. That's something to do with the way the game is being started up. Its getting confused at the stage when its using VESA. Are you starting UB in a way different to TR?

I start in exactly the same way as I do for TR 1. I load up Glidos from the Start Menu, select Unfinished Business from within Glidos (the disc is in the CD-ROM drive), and keep the same settings as for TR: 1280x1024 with forced smoothing. I feel that it's some issue with the 3dfx tombub.exe, but everywhere I've checked on the web, this is the only file that's supposed to work. This is correct yeah?

Here's the tombub I use.

Here's another thought. Look in the Glidos.ini file and see if the VESASupport setting is the same for both TR and UB.

Let's see now... [VESASupport: Yes] is in TR's settings. It isn't in UB's configuration listing. I have put it in and am about to test it...

Last famous words 😀

... oh yes. Inserting the VESA support line makes moi a happy bunny. All's well that ends well. Cheers all.