In 704 CE Bishop Wealdhere of London wrote a "letter close" , i.e. a letter for private communication, to Archbishop Brihtwold of Canterbury.
This letter, formerly in the library of Robert Cotton, and preserved in the British Library (Brit. Lib. MS Cotton August II, 18), is the earliest surviving
"letter known to have been written by one Englishman to another. . . . Although the letter has no dating clause, internal evidence shows that it cannot have been written earlier than 704, the year of Centred's accession to the Mercian throne, or later than 705, the year of Bishop Haedde's death" (Pierre Chaplais, "The letter from Bishop Wealdhere of London to Archibshop Brihtwold of Canterbury: the earliest original 'letter close' extant in the West", Parkes and Watson (eds.) Medieval Scribes, Manuscripts & Libraries. Essays presented ot N.R. Ker  3-4).