Calculating month movement and returning blanks

[JEB85]

Hi,

My powerpivot output is currently summarised as follows (the desired column is the result i would like):

[TABLE="width: 426"]
<colgroup><col style="text-align: center;"><col style="text-align: center;"><col span="3" style="text-align: center;"></colgroup><tbody>[TR]
[TD="align: center"]FinancialYear[/TD]
[TD="align: center"]MonthName[/TD]
[TD="align: center"]Balance Sheet[/TD]
[TD="align: center"]Movement[/TD]
[TD="align: center"]Desired[/TD]
[/TR]
[TR]
[TD="align: center"]2015[/TD]
[TD="align: center"]Sep[/TD]
[TD="align: center"]1,344[/TD]
[TD="align: center"]1,344[/TD]
[TD="align: center"][/TD]
[/TR]
[TR]
[TD="align: center"][/TD]
[TD="align: center"]Oct[/TD]
[TD="align: center"]1,570[/TD]
[TD="align: center"]226[/TD]
[TD="align: center"]226[/TD]
[/TR]
[TR]
[TD="align: center"][/TD]
[TD="align: center"]Nov[/TD]
[TD="align: center"]1,486[/TD]
[TD="align: center"]-84[/TD]
[TD="align: center"]-84[/TD]
[/TR]
[TR]
[TD="align: center"][/TD]
[TD="align: center"]Dec[/TD]
[TD="align: center"]1,526[/TD]
[TD="align: center"]40[/TD]
[TD="align: center"]40[/TD]
[/TR]
[TR]
[TD="align: center"][/TD]
[TD="align: center"]Jan[/TD]
[TD="align: center"]1,079[/TD]
[TD="align: center"]-447[/TD]
[TD="align: center"]-447[/TD]
[/TR]
[TR]
[TD="align: center"][/TD]
[TD="align: center"]Feb[/TD]
[TD="align: center"]1,480[/TD]
[TD="align: center"]401[/TD]
[TD="align: center"]401[/TD]
[/TR]
[TR]
[TD="align: center"][/TD]
[TD="align: center"]Mar[/TD]
[TD="align: center"]1,882[/TD]
[TD="align: center"]402[/TD]
[TD="align: center"]402[/TD]
[/TR]
[TR]
[TD="align: center"]2016[/TD]
[TD="align: center"]Apr[/TD]
[TD="align: center"]1,051[/TD]
[TD="align: center"]-831[/TD]
[TD="align: center"]-831[/TD]
[/TR]
[TR]
[TD="align: center"][/TD]
[TD="align: center"]May[/TD]
[TD="align: center"]1,880[/TD]
[TD="align: center"]829[/TD]
[TD="align: center"]829[/TD]
[/TR]
[TR]
[TD="align: center"][/TD]
[TD="align: center"]Jun[/TD]
[TD="align: center"]1,549[/TD]
[TD="align: center"]-331[/TD]
[TD="align: center"]-331[/TD]
[/TR]
[TR]
[TD="align: center"][/TD]
[TD="align: center"]Jul[/TD]
[TD="align: center"]1,691[/TD]
[TD="align: center"]142[/TD]
[TD="align: center"]142[/TD]
[/TR]
[TR]
[TD="align: center"][/TD]
[TD="align: center"]Aug[/TD]
[TD="align: center"]1,740[/TD]
[TD="align: center"]49[/TD]
[TD="align: center"]49[/TD]
[/TR]
[TR]
[TD="align: center"][/TD]
[TD="align: center"]Sep[/TD]
[TD="align: center"]1,764[/TD]
[TD="align: center"]24[/TD]
[TD="align: center"]24[/TD]
[/TR]
[TR]
[TD="align: center"][/TD]
[TD="align: center"]Oct[/TD]
[TD="align: center"][/TD]
[TD="align: center"]1,764[/TD]
[TD][/TD]
[/TR]
</tbody>[/TABLE]

The 2 measures i have calculated are as follows:
Balance Sheet =CALCULATE(sum(BalanceSheet[Accrual (£)]))
Movement =CALCULATE(sum(BalanceSheet[Accrual (£)]),PARALLELPERIOD('Calendar'[Date],-1,MONTH))-[Balance Sheet £]

So i'm calculating the month movement on the balance sheet.
My data set begins in Sep-15 and will be updated on a monthly basis - the last set of data i have is Sep-16.
I would like the first month's movement to return blank (Sep-15 in this case) as essentially it's calculating current month - prior month balance sheet when there is no prior month's balance sheet.
Similarly, i would like to return a blank in Oct-16 as it's not a real value (i don't have any data for Oct-16).
The only complication is that my data set is at a product level and we have new products come in each month so if i pivot by product i don't want to return a blank if the product didn't exist in the prior month.

Is there a way to return the blanks i desire?

Thanks