Hi there and thanks very much for your reply. I'm pleased to say that the sample you linked does open in 8.2.6.6 for me and more importantly it will now 'save as' to both .png and .jpg correctly, which was my issue with the previous IG release version.
I will test with further CR2 images that were supplied to me, but this suggests that maybe there was some file corruption with the others I had an issue with. That being said, they did open as expected in the Windows Photos app. However IG remained on a black screen with no 'loading' message and no error.