Active Customers in Financial Year

Hi

I have been wrestling with this issue all weekend and it should be pretty straightforward however I must be missing something.

I am building a report to show total customers in any financial year & I have already built these first measures successfully:

CUST 2016-17 =
            VAR CustomerCreation = FILTER(CUSTTABLE,CUSTTABLE[CREATEDDATETIME] <= DATE(2017,03,31))
            RETURN
            COUNTROWS(CustomerCreation)

This brings me back to total number of customers.

I then need to establish over that particular financial year how many of these customers actually purchased.

I already have a last sales date measure:

Last Sales Date = LASTDATE(SALESLINE[DELIVERY DATE])

So my logic is I use this within a date filter to count the respective customers, so I have written:

Active Customers =
   VAR TotalActive = CALCULATE(DISTINCTCOUNT(SALESLINE[CUSTACCOUNT]),FILTER(SALESLINE,SALESLINE[DEPOT] = "UK"))
    RETURN
    IF([Last Sales Date],DATESBETWEEN('DATE'[Date],01/04/2016,31/03/2017),TotalActive,BLANK())

I must be missing something and I cannot see it as the variable returns all unique customers in the UK, then the IF statement should apply the dates filter and return the DISCTINCTCOUNT stored in the variable or BLANK.

Any help would be appreciated.