Thanks for your response.
The only dimension is Month_Year (Not an actual date due to data issues but in format YYYYMM)
The following expressions:
Billed Amount (-sum(BILLED_AMOUNT))
Actual Spend (Sum(TOTAL_VALUE_EXCL_VAT))
Plan Monthly (MAX_FM_ACQ_BOOK_VAL_AMT/MONTH_BETWEEN)
Spend Accrual ([Plan Monthly]-[Actual Spend])
Balance ([Billed Amount]+[Actual Spend]+[Spend Accrual])
As i posted before, i believe i have cretaed a chart which correctly summarizes everything apart from the 'Plan Monthly' expression as this seems to return nothing or the total amount for the entire programme and not the selected months.