StanleyPain I dunno about "scariest ever" but it's damn good. It was a fave of mine as a kid. A friend of my moms, a woman she went to college with, wound up actually living in Avebury (where this was filmed) for about a year and she sent me tons of photos and postcards and I nearly freaked...I had no idea the place was real, much less the stone circles were real.
I went like 20 years or something without seeing it only to discover the original DVD release was region free and imported it. It aged pretty well I think. Sort of like a Nigel Kneale story geared towards kids, but only just slightly. The storyline is pretty mindblowingly metaphysical and weird for something geared towards young adults and, like the best creepy stories, it ends with more questions than answers. It's become something of a Halloween tradition with me now.
Old_Zircon 'The scariest programme ever made for children' is Jim'll Fix It.