Calculated Fields in a Pivot Table when 'Fields' appear over Muliple Lines

I have a Pivot Table where the 'Fields' appear over multiple lines, e.g. M1 Estimated Spend £000s appears as

M1
Estimated
Spend
£000s

I want to add some 'calculated fields' to give me specific totals and every thing I've tried to date just gives be an unhelpful message telling me there is an error in the formula. Basically if I wanted to say total the first three months estimated spend (see sample attached), I'd assumed this would work, but it doesn't

= M1 & CHAR(10) & Estimated & CHAR(10) & Spend & CHAR(10) & £000s + M2 & CHAR(10) & Estimated & CHAR(10) & Spend & CHAR(10) & £000s + M3 & CHAR(10) & Estimated & CHAR(10) & Spend & CHAR(10) & £000s

Note: There is a space both before M1, M2, M3 etc., and at the end of each line before the CHAR(10) line return apart from the last line where there is no space after £000s.

Is it me doing something wrong or is it simply not possible to achieve this when using line breaks in the field names?