The Disappearance of Melanie Hall

On October 5th, 2009 workmen were carrying out maintenance along the northwards side of the M5, clearing the undergrowth, when they came across some thin, cheap black plastic bin bags, tied across the top with building twine. Inside were bones and a skull. After thirteen years of fear over what had happened to Melanie Hall, the worst news was confirmed. These were the remains of her body. Although she had officially been declared dead, five years earlier in 2004, hope remained in her family that, by some miraculous chance, their daughter might turn up alive one day. That painful desire was wiped out by the discovery of those carelessly discarded bin bags.
The question remains...Who killed Melanie Hall?