A Scots song picked up in London in 1666 must have been extant, in one form or another, in Scotland prior to that date, the question being how long would it take at that period for a ballad/song to become so well known as to merit a mention by Pepys.

Fau"t would fit better than fault from a Scottish singers point of view.

Martinmas was the time of year when a "mart" ie. a steer was slaughtered and salted down to see the family through the winter and may have suggested to a Scots composer the mention of blood letting.