@arminunruh Thanks Armin I will take a look and test that too. Imgix is becoming really widely used, and offers a whole suite of image processing features at scale, so I'm keen to get that working - particularly to meet the needs of clients, rather than relying a plugin approach.
I've been in touch with Imgix tech support, and one comment they have made is that src value inside the image tags are base64 encoded, so Imgix and other similar services may not be able to intercept/recognise these particular images for processing. Not sure if that checks out or not, but I'd love to see if I can get at least get to the bottom of why it's not working.