This is a full-length novel from the late seventies or early eighties, set in a contemporary English city (possibly London). The protagonist is a man in his thirties, who has some mild ESP abilities (specifically, the ability to "pick up an aura"), and is the subject of a parapsychological research project.
He's captured by a small group of similar psychics, who are the (involuntary) servants of an alien that lives in their basement. The alien is a shapeless pink mass - when he first sees it, the protagonist says to himself "I've walked into the stomach of the house!" I believe it calls itself 'The Soft One' or 'The Formless One' or something of the sort. It eats keratin - the protagonist finds a dead bird whose feathers have been eaten, and there's a fairly graphic description of a human who has suffered a similar fate. It has considerable psychic powers, and causes the protagonist to experience realistic hallucinations to persuade him to become its servant - he's (apparently) transported to a small town in America, and (on a different occasion) murders his girlfriend.
The alien is a fugitive from justice, and an alien bounty hunter arrives in the vicinity of Earth, intending to destroy it. The bounty hunter plans to drop a bomb on England, which will kill millions - it's aware of the presence of humans, but regards them as insignificant as they don't have telepathic powers. The protagonist becomes aware of the bounty hunter in some way - it's possible that his powers, enhanced by The Formless One, enable him to intercept its transmissions to its home planet.
After overcoming various perils which escape my memory for the moment, the protagonist kills The Formless One by burning down the house. The bounty hunter leaves without dropping the bomb, and the protagonist is reunited with his girlfriend (who he didn't murder) and loses his powers completely.