Thanks Lyudmil,
Main reason for using the .eps was that this was how I saved the graphic originally – because it’s a logo I’m placing it in multiple other .ai files without a background, so it seemed like the best format to use. I was also advised that enlarging or shrinking a .png image reduces the image’s resolution, which doesn’t happen with .eps.
To import the file, I just used the menu in PowerPoint – Insert>Picture. I’ve since managed to find a workaround to more or less resolve the issue, but I still have no idea why it was happening in the first place.
Joseph