What is the standard of proof laid upon a defendant asserting accord and satisfaction?

The standard of proof resting upon the defendant asserting an accord and satisfaction was described as follows by Ritchie C.J. in Weldon v. Vaughan (1880), 1880 CanLII 34 (SCC), 5 S.C.R. 35 at 42: “As the burthen is on the defendant of establishing an allegation of accord and satisfaction, he is bound to establish it beyond all reasonable doubt, and if the evidence was verbal, and had to be submitted to a jury, it would be the duty of the jury to find against the defendant on an issue of accord and satisfaction, unless defendant’s evidence established it to their satisfaction beyond a reasonable doubt. So, if he relies on documents, which the court have to construe, as establishing his defence of accord and satisfaction, and they are so ambiguously worded as to be fairly capable of a construction inconsistent with his contention, I think the court, unless satisfied beyond a reasonable doubt that what is put forward as an accord and satisfaction was intended by both parties as such, and that there was an acceptance in satisfaction as an act of the will of party receiving, should not, by a doubtful construction, deprive a plaintiff of an unquestionable legal right which accord and satisfaction assumes he has.”