Hi Ewan
I agree with Rob, nice chub that. I think chub are one of the hardest fish to put a weight to, especially from photos, and they vary a hell of a lot for a given length.
My rule of thumb was always that they're never as heavy as you think they are, and if you want to put a weight to them then you need to weigh them. No point in guessing.
I wouldn't even think about a weight, just calculate the length from the photo and then talk about that. Or you can just say enigmatically and possibly with a tear in your eye, "aye, I remember that big fly-caught chub..."
That trout/mayfly pic is amazing.
W.