The comedian has reprised his character Geraldine McQueen - winner of Britain's Got the Pop Factor... and Possibly a New Celebrity Jesus Christ Soapstar Superstar Strictly on Ice - to duet with Boyle on a cover of Elaine Paige and Barbara Dickson's 'I Know Him So Well'. A video for the track will premiere during the BBC telethon tomorrow night.
"I just thought it would be a laugh and so I wrote to Susan Boyle and she was really up for it," Kay told The Bolton News. "She was great on the day and worked very hard. We filmed at studios in Salford and carried on from 8am to 1am the next morning.
"We stuck very closely to the original and when you see the Paige and Dickson version, you can see how similar they are. We have obviously added some funny bits in, but they are fairly subtle - I think it works better that way."
He added: "I hope people will enjoy it and that it raises lots of money for Comic Relief."
Appearing as McQueen, Kay will also join the Loose Women lineup next week as their first 'transsexual' panellist.
"Geraldine is going to be perfect," a source told The Mirror. "Her larger-than-life personality and straight-talking attitude makes her ideal."