What is the test for establishing that “the something” put forward by the defence is a reasonable excuse?

Lord Parker C.J. in Law v. Stephens, supra, held that once “the something” put forward is found capable, in law, of being a reasonable excuse, it then becomes “a matter of fact and degree as to whether or not it amounts to a reasonable excuse.” At this stage (the third) in the process, the judge, if necessary, will make determinations about the credibility of witnesses, weigh the evidence, and settle questions of primary fact and inferential fact, all geared to deciding the substantive merits of the defence. These are all questions of fact and the burden respecting them, as Lord Parker C.J. points out and the last branch of s. 794(2) provides, is on the Crown to negative them as working in favour of the defendant.