I can't think why anyone should want to connect a rad in that way Andy. Has it worked ok in the past?
Technically it's alright, but the valves should be located diagonally, so you would have inlet bottom left, and outlet top right. This would give a proper crossflow. If a rad was, say, 1800mm long but only 500mm high, then I can't see how the "other" end would ever get any hot water.
Anyway........... cold at the TOP normally indicates bleeding needed....... but, with your arrangement, all bets are off. Gawd knows where the cold spot would be with this set-up. Try a bleed.