In secondary 2/3 of their GCSE is with a calculator. Their mental maths doesn't slow them down here. Maybe there is a place for calculators at primary to engage children into the processes of maths regardless of the numbers and supporting then to understand the rules of number.

I think ‘mental maths’ is more an automaticity, a deep understanding of mathematical structures and a fluency, where you can ‘see’ in your head the structure of the problem and what methods or procedures you might choose to solve it… with your calculator.