Please note I have had permission from S&S to add links to my website to the answer books:
Book 2:
https://www.schofieldandsims.co.uk/downloads/MentalArithmetic/mental-arithmetic-2-answers.pdf
Book 3:
https://www.schofieldandsims.co.uk/downloads/MentalArithmetic/mental-arithmetic-3-answers.pdf
Book 4:
https://www.schofieldandsims.co.uk/downloads/MentalArithmetic/mental-arithmetic-4-answers.pdf
Book 5:
https://www.schofieldandsims.co.uk/downloads/MentalArithmetic/mental-arithmetic-5-answers.pdf
Book 6:
https://www.schofieldandsims.co.uk/downloads/MentalArithmetic/mental-arithmetic-6-answers.pdf