I've just checked an Lp I thought the vocal may have been on, 'Sufferer' by the Kingstonians, but no luck, it was something else that the Lp The Undertaker and that Lp share.
I read the cd notes of the JA Gold Undertaker release where Harriot says that DR. Who was recorded especially for that Lp and was one of the first tunes cut.
Of course It may have been used for a vocal track later. It's very reminiscent of one or two of the basslines on the 'Sufferer' Lp though, maybe you should give that a listen to check your theories?
Someone on here should be able to help you, Try Contacting Charlie Reggae with a PM or e-mail, he's very knowledgable about stuff of this type, and gave me the Sufferer Lp too... nice bloke that he is..