Heather laid down lyrics for the track she named "Disability Rap" soon after she began dating Paul in 1999.
In the song, Heather says: "It's been hard, other children can be cruel. Teasing and tormenting - freak, weirdo, retard. What do they know?"
"Disability Rap" also features Paul's vocals and Heather wanted the song to be released to raise money for disabled children.
A source told Britain's News of the World newspaper: "Her brassy voice is sometimes technically altered as she awkwardly raps over techno music."
"Then Paul takes over with sweeping orchestral backing, but not even his talents can save it."
"Paul hated the idea, but Heather thought it was the greatest record she had ever heard."
In the song, Heather - who lost her left leg below the knee in a motorbike accident in 1993 - also blasted the British health service, the NHS, for failing to provide adequate prosthetic limbs.
She raps: "So you go to the NHS and they do their best with their limited budget. They make her limbs but she can't wear them - they're painful. And every step you take is even more painful to you."
"So you thank them because it's the best they can do. No Mr. Minister. It's not. And you know it."
