In 1066, when the Normans invaded England, they brought two languages with them; French and Latin. Latin was mostly used for written language, especially that of the Church. Gradually, these languages mixed with the Anglo-Saxon and Viking tongues to form English, although the influence was mostly in terms of vocabualry. During various periods of expansion of vocabulary, such as the Elizabethan age, Latin has been used as a source of neologisms.
I would like to add to what TDOL said. Even though the Romans had occupied most of Great Britain for a long time, very little residual Latin remained after Caesar removed his troops. There are only a couple of English words that came from Latin in that period. The Romans did not believe in educating their conquests at that time. So the Latin influence came, as TDOL said, with the French Normans and with the clergy.