Dr. Matthew Happold of the law faculty at the University of Hull writes us with this considerably more detailed treatment of the pardon appeal on behalf of the Scots medium Helen Duncan:

As an English lawyer, I am, of course, always happy to traduce Scots law, but the situation was a little more complex than you suggested. A considerable number of prosecutions for witchcraft took place in early modern Scotland (some 3,837 people according to the University of Edinburgh’s Survey of Scottish Witchcraft) and torture was used up until 1704. However, in 1735 Parliament enacted an Act (long title: “An Act to repeal the Statute made in the First Year of the Reign of King James the First, intituled, An Act against Conjuration, Witchcraft, and dealing with evil and wicked Spirits, except so much thereof as repeals an Act of the Fifth Year of the Reign of Queen Elizabeth, Against Conjurations, Inchantments, and Witchcrafts, and to repeal an Act passed in the Parliament of Scotland in the Ninth Parliament of Queen Mary, intituled, Anentis Witchcrafts, and for punishing such Persons as pretend to exercise or use any kind of Witchcraft, Sorcery, Inchantment, or Conjuration”), which repealed the 1563 statute which had previously governed
prosecutions for witchcraft in Scotland.

The 1735 Act was premised on Enlightenment beliefs in the non-existence of witchcraft. It provided that, from its coming into force, “no Prosecution, Suit, or Proceeding, shall be commenced or carried on against any Person or Persons for Witchcraft, Sorcery, Inchantment, or Conjuration, or for charging another with any such Offence, in any Court whatsoever in Great Britain.” Instead, “for the more effectual preventing and punishing of any Pretences to such Arts or Powers …, whereby ignorant Persons are frequently deluded and
defrauded … any Person [who] shall … pretend to exercise or use any kind of Witchcraft, Sorcery, Inchantment, or Conjuration, or undertake to tell Fortunes,
or pretend, from his or her Skill or Knowledge in any occult or crafty Science,
to discover where or in what manner any Goods or Chattels, supposed to have
been stolen or lost” could be subject to criminal sanction. Possible penalties
were imprisonment for up to a year, exposure in the pillory and being required
to find sureties for good behaviour, but not death. Essentially, persons
claiming to have magical powers were to be treated as fraudsters.

It was under the 1735 Act that Helen Duncan was convicted. Indeed, much of
the
clamour by spiritualists against her conviction arise from their belief that
she
was not a fraud; that her powers were genuine, and that she was punished for
telling the inconvenient truth (although it’s also true that by the date of her
prosecution the 1735 Act had fallen into disuse).

Interestingly, in 1951 the 1735 Witchcraft Act was itself repealed and replaced
by the Fraudulent Mediums Act, which provides for “the punishment of persons
who fraudulently purport to act as spiritualistic mediums or to exercise powers
of telepathy, clairvoyance or other similar powers.” Some might say this was a
regression, rather than progress, as it could be interpreted to suggest that
some mediums are not fraudulent. That might, however, be simply because it is
considered that they delude themselves, rather than that they exercise genuine
powers.