They came back to say that the quality of the upload of a pdf scan of my passport isn't good enough. It actually looks ok to me, but my photo is in black and white (I don't know why when I submitted a colour photo to the passport office) and I have silver grey hair, very pale skin and it's against a white background so I think the problem might be that it's just eyes, eyebrows and a mouth with no defining edge. I'm not sure that there's much that I can do about it. I'm going to re-scan to see if it can be a bit better.