Thanks, Ed. So let me rephrase the question. Has Boxing Day moved between Sunday and Monday when the 26th falls on a Sunday:

a) always, as far as we can tell?

b) seems to have been introduced many years ago (with this evidence of it happening in the - for instance - 17c)

c) appears to be a recent introduction (with this evidence of it being on the Sunday in - say - the late 20c)