>> The thing is, we simply don't know when that linguistic result of the
>> 'occupation' took effect; in a short while due to an invasion, or over a
>> very long time? When exactly did all the inhabitants of, say, Essex or
>> Surrey, speak a form of English and no longer a form of Brythonic? After
>> 650 AD or even after 800 AD?
It's difficult enough to work out what happened to Cornish let alone
Brythonic in Essex. Dr Ken George has estimated that in 1200 93% of
Cornwall was occupied by Cornish speakers but that by 1500 people spoke
Cornish in 54% of Cornwall. However Dr NJA Williams says that by the mid
16th C Cornish was still spoken as far east as the Tamar. The retreat of
Cornish may not have been as quick or as complete as many think. However
it did retreat westwards without any mass immigration of English people.
Despite this Cornwall managed to stay very Cornish, probably until the
arrival of the railways after which things began to change rather faster.