When they presented SM:tv, they had a segment called Challenge Ant, where the one called Ant was quizzed mastermind stylee in a big chair. I remember the segment so have always been able to identify Ant, so have always recognised Dec, by the fact he's not Ant.
Ant has the larger forehead.
I've been watching George R Martin videos on youtube where he reveals what real historical events he based plot points on. Most of the really violent stuff seems to be Scottish or French history, which explains why I was so bored studying the wars of the Roses at school, but love game of thrones.