I seem to remember reading that there was a conscious effort by the Normans to supplant the Anglo-Saxon language with Norman French from the beginning, especially in the law courts. It was part of the divide and conquer process which Willy the tanner's by-blow from Falaise put into practice so effectively from 1066 onwards. By 1087 the use of French in the courts was more or less mandatory. 20 years is not a long time in linguistic terms!

The Romans, on the other hand, couldn't give a damn about the language issue as long as the local tax farmers kept them cards and letters rollin' in ...