The Liquidator is chosen as a tribute to the winners of the UEFA Champions League. It is a popular tune to play as football teams run out, and Wikipedia says that Chelsea, Wycombe Wanderers, Northampton Town West Bromwich Albion, Wolverhampton Wanderers and St. Johnstone all have claims to have been the first club to use it.
But that same entry suggests that Chelsea's is the strongest claim, quoting the liner notes for Liquidator: The Best of the Harry J & The All Stars:
"Way back in 1969, supporters of the Chelsea football team revered players such as Bonetti, Osgood and Hollins. The boys performed under the watchful eye of manager Dave Sexton to the tune of Harry J & All Stars chartbuster, 'The Liquidator'".Harry J was Harry Zephaniah Johnson, who later enjoyed success as a record producer, familiarising Britain with reggae through the gentle hands of artists like Johnny Nash and Ken Boothe,