when was "well" first used to begin a sentence in england

In writing the supposed memoirs of a late eighteenth century widow I'm wondering when "Well" at the beginning of a sentence was first known to have been used.

Solution 1:

OED (paywalled) has it used as an interjection in Old English, and has a substantive citation in Middle English from the fourteenth century:

well, adv. and n.4

V. In disjunctive use (sometimes as a simple interjection).
20. Used to introduce a remark or statement, sometimes implying that the speaker or writer accepts a situation, etc., already expressed or indicated, or desires to qualify this in some way, but frequently used only as a preliminary or resumptive word.
_{Well functions as a discourse marker, often expressing an emotion such as surprise, indignation, resignation, or relief, but also used when pausing to consider one's next words, to introduce an explanation or amplification, to mark the resumption or end of a conversation, etc., or to indicate that one is waiting for an answer or explanation from someone.}

a. Not combined with another expression.
_{In Old English apparently only after wel lā int. with intervening noun (as vocative); see wella int. 1, and cf. discussion in etymology.}

eOE King Ælfred tr. Boethius De Consol. Philos. (Otho) (2009) I. xxxi. 531 Wella wisan men, wel, gað ealle on þone weg [etc.].

c1330 Sir Tristrem (1886) l. 2950 (MED) Wel, whi seistow so?