Hi Jeff,
Thanks for your response.
Re-interpreting the footage was one of the things I tried, yes, but that didn’t do it for me. However, I’ve since learnt that, in this instance at least, the aspect ratio was essentially irrelevant. It was the codec involved in the clip that was the issue – to generate the correct sequence settings for my AE animated file I had to create a custom preset. Upon creating a new sequence I set it to Digital SLR 1080p25, then went to the general tab, changed editing mode to Desktop, then the video preview file format to Animation, ensuring the dimensions were 1920 x 1080, then saved the preset. Premiere then treated the footage as it should.
I’m sure this is no doubt an amateur mistake, but maybe this info will prove useful to someone else down the line, since all my troubleshooting was focused in the wrong place, i.e the aspect ratio.