Cheers folks.
Here's what I was doing - exporting website page I was testing this on and then uploading it to ftp. Size of image was 1200 x 630 png. I then followed Gary's advice (thank you, as ever!) and made the image 1200 x 627 jpg and published the site (rather than export) and all works beautifully.
Not sure which part of my original workflow was the issue, but you solved it.
Thanks m