Last Night's TV: The Pendle Witch Child/BBC4<br />Who Do You Think You Are?/BBC1

As hills go, Pendle, in Lancashire, surely punches above its weight when it comes to looming. A long un-risen loaf of moorland, it isn't particularly high or particularly distinctive in shape. And yet it has an undeniably foreboding presence in the landscape, accentuated on screen by the fact that it's very rarely depicted without a heavily filtered sky. They got so carried away with the effect in The Pendle Witch Child, in fact, that there were times when it looked as if it was wearing a black cap of overcast – not inappropriate given the fact that quite a few of the film's subjects ended up dangling from the end of a rope.

It's hard to know whether Pendle acquired its air of menace from its connection with the witch trials described in Simon Armitage's film or whether its brooding topography helped give rise to the panic in the first place. Certainly the connection is well-established now and lucratively exploited by local knick-knack shops and cafés. Armitage's purpose was to strip away that touristic levity and restore a bit of terror to the story, as well as to show just how much damage a scary story can do when it finds an over-anxious audience.

The child of the title was Jennet Device, a nine year old who, for reasons that still aren't clear, ended up denouncing her mother and nine others for witchcraft after her sister Alizon had convinced herself that she'd cursed a local pedlar into a state of paralysis. An ambitious local magistrate, anxious to make a name for himself, took up the case, inspired in part by a royal bestseller, King James's Daemonologie – a sort of "Witchhunting for Dummies" which offered a checklist for guilt and the helpful advice that children's testimony could often be invaluable in securing a conviction. Jennet, the prosecution's star witness, haunted the tale, both in Armitage's narrative and on screen, where she was summoned up in spectral, hollow-eyed form by Phoebe Boswell's superimposed animations.

Jennet's family paid an immediate price for her story, while she paid a deferred one – sent to the dock herself years later after another child, Edmund Robinson, had excused his late arrival home with an ad hoc tale of shape-shifting greyhounds and witchy kidnapping. He then toured the local churches denouncing various members of the congregation as paid up Satanists. Jennet's earlier testimony provided a case law precedent for the admission of his, as detailed in Dalton's The Country Justice – a standard handbook for magistrates at the time. Fortunately for her, King James was no longer on the throne and the mood of the magistracy was a good deal more sceptical and forensic. The famous physician William Harvey was called in to examine the accused for marks of the Devil (he found nothing "unnatural") and Edmund was eventually subjected to a stiff cross-examination, under which he buckled and admitted that it was all nonsense, generated in part by the lurid stories he'd been told about the earlier case.

There was a sting in the tail, though. Years later it was a copy of The Country Justice that was resting on the desk of the judges during the Salem witch trials in Massachusetts, another notorious instance of juvenile denunciation. And just in case we got too complacent, Armitage reminded us that social panic and judicial overreaction have never gone out of fashion – a timely note as English courts work overtime to process our own modern spirits of unrule, in their hoodies and looted trainers.

The standard unit of measurement for an episode of Who Do You Think You Are? is the OMG (the Oh My God) – though readings always have to be adjusted for the excitability of the participant. One of the highest OMG ratings ever recorded was for an episode featuring Sarah Jessica Parker, but since she even appeared to be startled to find that she'd had a great-grandmother, most reputable tables exclude her. J K Rowling was less easily amazed by the basics of genealogy, but even so her programme had a very respectable OMG figure. We got one when she discovered that her maternal great-grandfather shared a birthday with her (and so, by extension, with Harry Potter), another when she found out that he'd won the Croix de Geurre for a heroic rearguard action during the First World War ("I've always been most impressed by bravery against the odds"), and a bleak and melancholy one when she learned that his remains now lay in a common grave, probably unrecoverable even with her considerable resources.

t.sutcliffe@independent.co.uk