TheGiantDonut: Just a quick reply to say that I contacted GOG support last week and followed their advice, and now the game is launching properly and running well. Very good support!

I think the actions I had to take were specific to my system, so I won't post the details here - but in summary, it involved:

- reinstalling the game as an administrator
- installing some DirectX and graphics card (nvidia) updates
- launching the game as administrator

So I don't know if was just one, some or all of these actions that enabled my PC to run the game.

I advise anyone who has the same problem as me to contact GOG support.