Both sprung from the uprising of an oppressed people against a tyrannical and arbitrary government who continuously violated the common law with fiat law.
But the question was "What is the source of the common law?" You said judges, which clearly wasn't the case during the creation of the Magna Carta. The nobles themselves overruled the King at the points of a number of spears.
From my viewpoint, that's an example of common law in action, regardless of how the lawyer class tries to exclude us mere peons from any "lawmaking".
The nobles themselves were the judges. It should be kept in mind that all of this came about through a somewhat continuous process of refinement. "
"continuously violated the common law with fiat law. "
There is nothing particularly special about either of those, they are not absolute, nor are they fundamental. What is absolute is the concept of natural law, which is fundamental. Natural law contains the concepts of Freedom, rights that protect all individuals and their nature equally. Common law contains nothing within itself, it is just a process and collection of judgements and decrees.
"The nobles themselves overruled the King at the points of a number of spears. "
They did this with help from those that received favors from the nobles. If there is any valid justification for what they did, it is to be found in natural law, not common law. Contemporary attempts at corruption use the socialist foundations, such as those created by Rawls. Establishing common law based on those leftist foundations is a simple recreation of the serfdom that was eliminated by virtue of recognition and acceptance of natural law in a long process.
It's the underlying principles that count, not the process.