Behind them, fenced off with barbed wire, the crumbling stone obelisk of the Hagley Hall estate teetered heavenwards, as it has done for 200 years. On it, a sinister piece of fresh graffito gleamed in the half-light: "Who put Bella in the Witch Elm?"
For Hagley-dwellers - and especially for those who remember the village before the post-war expansion of Birmingham forcibly connected it to the modern world - those words have a dark significance. They refer to a story which retains an unsettling force in these parts.
It begins on a sunny April Sunday in 1943, when four teenage boys from nearby Stourbridge went birds'-nesting in Hagley Wood. Their quest took them to an old, hollow wych-hazel - also known as a wych-elm, on account of its size and age. For a minute or two they climbed and searched. Then one of them, Bob Farmer, gave a cry: from out of the tree, a white skull was grinning at him. "There was a small patch of rotting flesh on the forehead with lank hair attaching to it, and the two front teeth were crooked," he later stated.
The frightened boys ran away and - unsure if the skull was human or animal but certain that they should not have been in the woods in the first place - at first told no one about their find. Then Tommy Willetts, the youngest, told his father, who told the police. Their investigation uncovered the more-or-less entire skeleton of a young woman within the tree. Her mouth was stuffed with taffeta, and a gold wedding ring and some crepe shoes were found nearby - but perhaps most chillingly of all, one of her hands was missing. This, it was suggested, was a classic sign of a black-magic execution.
Subsequent examination - by the well-known forensic scientist Professor James Webster - suggested that the dead woman had been 35, was the mother of one child, and had been dead for about 18 months before she was found. The coroner declared it murder, with asphyxiation the probable cause. Exhaustive trawls through dental records and missing-person files proved unexpectedly fruitless, and the press were briefly enthralled. But, before long, the horrors of war distracted attention from the "Tree Murder Riddle".
But then, that Christmas, the graffiti began to appear. "Who put Luebella down the wych-elm?" said the first one, in nearby Old Hill. "Hagley Wood Bella", said another, in Birmingham. Gradually, the messages - which seemed to be written by the same hand - took what was to be their settled form: "Who put Bella in the wych-elm?" they asked.
The implication was that somebody knew, but appeals to the unknown graffitist to contact the police proved fruitless. Instead, the slogans continued, and, at some point in the late Forties, other hands took up the cry as well.
Like "Kilroy was here", it soon became a slogan for those who could not think of a slogan of their own, an expression of solidarity with fellow graffitists, with a hint of contempt for urban sophisticates who weren't in on it. "It's just a joke," says Carl Baldacchino of West Mercia police. "It doesn't mean anything."
In this case, though, there was a difference. Whether or not the graffitist knew the answer, an answer was still required: who did put Bella in the wych-elm?
The general use of the name Bella, a common Black Country name, to describe the unknown victim gained currency when Professor Margaret Murray, of University College, London, made the suggestion that the severed hand was a sign of a black-magic execution. Belladonna is associated with witchcraft, as is wych-hazel, and as was, according to local legend, Hagley Wood. The "witch" theory - that Bella had been executed for unspecified crimes against a coven - quickly became popular.
But in 1953, a rival theory - this one involving spying - arose to challenge it. Wilfred Byford-Jones, a columnist on the Wolverhampton Express & Star, wrote about the 10-year-old case and was contacted by a woman who called herself "Anna" and claimed that Bella had been murdered for knowing too much about a pro-German spy ring which included a Dutchman, a foreign trapeze artist, and a British officer who died insane in 1942.
Rumours that two German parachutists had landed and vanished in the area in early 1941 lent weight to the "spy" theory, as did the plausibility of the spy ring's alleged activities: guiding Luftwaffe bombers to the various munitions factories in the area.
According to Byford-Jones, some of Anna's facts were subsequently "verified", and both MI5 and the police investigated her claims. But no arrest followed. Either it was a false trail, or someone was covering up.
At this point, the story frays away into loose ends. The mystery lay dormant for years. Every decade or so, a new outbreak of graffiti, or a new theory, would reactivate it. Yet new evidence was scarce. Some anonymous letters to another local journalist in the 1970s briefly revived the "spy" theme, but the allegations largely rehashed Byford-Jones's, while the suggestion that Bella was a Dutchwoman called Clarabella Dronkers was never confirmed.
The odd detail lent weight to the suggestion that people in high places were suppressing evidence: it emerged, for example, that Professor Webster, now dead, had bequeathed Bella's skeleton to a friend at Birmingham University Medical School, but that it had somehow gone missing. However, the passage of time was already making certainty impossible. West Mercia police refused, and still refuse, to allow anyone access to their files, on the grounds that the case is still open. The cover-up theorists rest their case.
But no one has yet suggested who is being protected, or why, and the fact remains that none of the more colourful lines of inquiry has yet led anywhere. Alternative explanations - that the killer may have been a GI (perhaps the father of that one child), and that Bella may have been abducted (in her taffeta nightdress) when she fled from an air-raid that subsequently destroyed her Birmingham house - have repeatedly been advanced. And, while they're neither provable nor satisfying, they're closer to the received wisdom in Hagley than either the "spy" or the "witch" theory. After the last outbreak of Bella fever (in the mid-Eighties), it really looked as though the lady in the wych-elm would be left to rest in peace. Can the new graffito on the obelisk revive the old misgivings?
At nightfall, Hagley Wood is a place of dark rustlings and confusing shadows. The comforting murmur of Birmingham and its motorways is always just audible, but it doesn't take much to imagine yourself back into the vast, rural silence of the past. An explosion of broken twigs - caused by a family of startled deer - can still make even a cynical heart beat faster.
The remains of Bella's wych-elm are still there, rotted by age and buried deep among brambles and thrusting sycamores that have grown tall since her day. Is black magic performed around this wrinkled crone of a tree, whose shock of suckers has been compared to a witch's spiky hair? If so, I hope that those who perform it are wearing some good thornproof clothing.
But no one I asked in Hagley - from a self-professed paranormalist to a respected local historian - could tell me anything of a current occult tradition involving Hagley Wood. "If it was witchcraft, it's the only incident of its kind that I've heard of round here," says Geoff Pardoe, Hagley representative of the Worcestershire Local History Forum. "I've never come across any of it," says Harry Tromans, a former Daily Mirror journalist who wrote about the Bella case as a cub-reporter in 1943.
Later, in the archives of the Black Country Bugle, I found old letters suggesting that, before the war, witches' sabbaths were regularly held in Hagley Wood, while the pub opposite, The Gypsies' Tent, was associated with hauntings and other occult goings on. But The Gypsies' Tent has long since made way for a Travel Inn, and the authors of those letters are dead, as, indeed, are all who were directly associated with the mystery.
"It's a thorny old chestnut," says Tromans, stirring the arboreal metaphor with the conscious panache of an old hack. "There are all these theories, but no one actually knows anything. We'll probably never know. Anyone who might have known anything is dead, anyone who might have done it is dead. It's lost - we should let it go."
But whether Bella will be let go is a different matter. Lord Cobham, who owns Hagley Hall, has no plans to remove the graffito from his obelisk, for fear that "a power-wash might knock it down". As far as he's concerned, it's just one more example of a seemingly unstoppable vandalism problem that plagues his entire 1,200-acre estate, resulting from its easy accessibility to every urban delinquent in the West Midlands.
Yet visiting vandals from nearby Birmingham are unlikely to have been responsible for a message whose full resonance is appreciated only by that dwindling number of elderly Hagley-dwellers who have lived there since the Second World War. And, as long as the message remains for all to see, people will wonder what dark thoughts animated the elderly hand that wrote it.