No, I've worked it out and the upstairs are also undersized.
As I said, the installer did no assessments beforehand so no such thing as a heat loss calculation..
He is now saying that upstairs are not undersized and should provide you with heat loss calculations to do this. Ironically the heat loss calcs you say you have done you mentioned using the inner temp as 10 degrees, the calc should be based on the ideal temp as opposed to actual temp. So if you have done it the way you have indicated the size you worked out could also be undersized and yet your calculations still tell you you need larger rads than provided.