"As we have seen the basis of Common Law was custom. The itinerant justices set out by William the Conqueror examined the different local practises of dealing with disputes and crime, filtered our the less practical and reasonable ones, and ended up with a set of laws which were to be applied uniformly throughout the country."
Happy to assist your education.
laws are MADE UP by the courts and judges from these basis.
you seem to confuse the basis of law with the law itself. Or that the mere existence of these basis for law was not enough, and the codification of Common Law still required an additional step of MAKING UP the law from the basis.
from your own quoted text ... "As we have seen the basis of Common Law was ..."
if customs and local practices IS Common Law, maybe the very text passage you quoted should have skipped over "the basis of" part?