The 49-year-old presenter, actress and singer and the West End star have released a cover of the musical hit from 'Chess The Musical', which is set to feature on the former's upcoming debut album, 'Songs From My Heart', which follows on October 2.
The original was performed by Olivier award-winners Elaine Page and Barbara Dickson in 1985.
The pair have become the best of friends since recording together.
On how the duet with Sheridan, 39, came about, Amanda said: ''I got a great friendship out of this song in Sheridan Smith.
''I'd loved her for years and we've always hugged when we met but we didn't know each other. I really wanted her to duet with me on 'I Know Him So Well' so I asked David Walliams for her number. She was heavily pregnant, but she couldn't have been nicer or more enthusiastic when I called her, and the final result is better than I even imagined.
This is also the first single I ever bought for my mum who loved Barbara Dickson. We had all her albums and even though I have never to this day seen 'Chess', this is the song I've always loved.''
'Chess' features music by ABBA's Benny Andersson and Björn Ulvaeus.
Amanda dropped single 'With You', which is from 'Ghost' and pays tribute to her late son Theo, who was stillborn in 2011, earlier this month.
She said: ''People think they know who I am because they see me being loud and outrageous on the television or radio.
''That is a part of me but really I'm all about my family. This album is all the songs I've loved in my life sung for the people who mean the most to me. It's a very emotional album that everyone can relate to.
''I had a tape recorder as a little girl and would sing into it dreaming of this moment.
''There is a story behind every song on the album celebrating life, love and loss - the darkest and happiest moments in my life so far. It is literally my heart laid bare.''
Amanda has covered a number of musical hits on her debut album, including Les Miserables' 'I Dreamed A Dream', 'Not While I'm Around' from 'Sweeney Todd' and Evita's 'Don't Cry For Me Argentina'.
Other tracks, such as 'When She Loved Me' and 'Lullaby' remind the 'Britain's Got Talent' judge - who has two daughters Lexi, 14 and Hollie, eight - of the ups and downs of being a mum, from the unconditional love to the sadness, and everything in between.
The 'Cutting It' star released her first single, 'Over the Rainbow', in May to help raise vital funds for the coronavirus crisis.
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