When I installed PST initially, I got a pop up enquiring bout resolution which I used to set it to 1280 * 800 which ran OK. I wanted it on a different drive, so I uninstalled/reinstalled.This time no pop-up and it runs at some 4*3 resolution in the middle of the screen. I can't see any way to change it or even find out what it is? Any way to use the game as supplied by GOG o fix this, or do I need to get into MODs. I tried running setup-ddrawfix.exe but that seems to have made no difference. Help anyone?

The game's default resolution is 640x480. AFAIK, the GOG installer doesn't include any resolution/UI mods, so that's what you see; it's just your video card doing the scaling... or, in this case, not doing it. Not sure about the pop-up you got. You can make the game fill the screen vertically by tinkering with the video card settings so the picture is scaled while preserving the aspect ratio.

Then again, you probably do want to get into mods anyway (for bugfixes), so you could install the widescreen mod and let it handle the resolution. (Despite its name, the target picture size doesn't actually have to be "widescreen", you can set it to whatever resolution you want, as long as your hardware supports it.)