Thanks :p
The Magna Carta is in fact statutory law, which is the highest authority of law. Common law can be overridden by statute law. Magna Carta did set out some major constitutional law amendments though (trial by peers etc) but common law is important.
Self-defence isn't in a statute therefore the common law is the only authority on it we have (currently, I think). It is definitely one of the strongest defence because it either acquits or it doesn't.
I wish AeroScap was here, he's much cleverer than me at this sort of thing :p