I know it used to be A Whole Lotta Love by Led Zep but on this site it says..
"Theme Music
A version of Led Zeppelin's "Whole Lotta Love" by CCS was used as the shows theme tune for most of the 1970s although ironically the band never performed on the show. In 1981 an original song 'Yellow Pearl' by Phil Lynott was commissioned as the new theme music. This was replaced in 1986 with "The Wizard", a composition by Paul Hardcastle. The theme is currently a remixed version of the one used between 1991 and 1995. "