Complex sentence: In an adjoining studio, John Lennon was working on his album "Mind Games", and was about to use a harmonium, which Slade's producer Chas Chandler decided the song needed.
Split into simpler sentences: In an adjoining studio, John Lennon was working on his album "Mind Games". He was about to use a harmonium, which Slade's producer Chas Chandler decided the song needed.