Hi Vincent,
Thanks for your reply....
Missing tables/queries here goes....
qryMonthSalesCust was the original query posted =

SELECT DISTINCT qryStatementLine.AccountIndex, Format([InvoiceDate],"mmmm")
AS MonthName, Sum(qryStatementLine.Debit) AS SumOfDebit, DatePart("m",
[InvoiceDate]) AS MonthNumber
FROM qryStatementLine
GROUP BY qryStatementLine.AccountIndex, Format([InvoiceDate],"mmmm"),
DatePart("m",[InvoiceDate])
ORDER BY qryStatementLine.AccountIndex, DatePart("m",[InvoiceDate]) DESC;

qryStatementLine =

SELECT DISTINCT A.AccountIndex, A.AccountName, SO.CustomerOrderNumber, SI.
InvoiceDate, SI.SalesInvoiceNumber, SI.TotalNett, V.VATValue, CCur([TotalNett]
*(([VatValue]/100)+1)) AS Debit, T.TypeOfPosting, IIf([TypeOfPosting]
="Customer Payment",[TransCredit],0) AS IFFCredit, T.DateOfPosting, [Debit]-
[IFFCredit] AS ItemBalance, SI.SalesInvoicePaid
FROM tblVAT V INNER JOIN ((tblAccount A  INNER JOIN tblSalesOrder SO ON A.
AccountIndex = SO.AccountIndex) INNER JOIN ((tblDespatch D INNER JOIN
tblSalesOrderLine SOL ON D.SalesOrderNumber = SOL.SalesOrderNumber) INNER
JOIN (tblTransaction T RIGHT JOIN tblSalesInvoice SI ON T.TransactionNumber =
SI.TransactionNumber) ON D.DespatchNumber = SI.[Despatch Number]) ON (SO.
SalesOrderNumber = SOL.SalesOrderNumber) AND (SO.SalesOrderNumber = D.
SalesOrderNumber)) ON V.VATRate = SOL.VATRate
ORDER BY A.AccountIndex, SI.SalesInvoiceNumber;

tblDespatch = DespatchNumber (PK), SalesOrderNumber (FK for tblSalesOrder),
DateOfDespatch, (there are three other fields althought these are only
'flags' for coding)

tblSalesInvoice = SalesInvoiceNumber (PK), DespatchNumber (FK for Despatch),
InvoiceDate, TotalNett, CarriageAmount, TotalIncVAT (I know I shouldnt store
calculated fields, but storing it really does make the query building easier),
TransactionNumber (FK for tblTransaction, no referential integrity as a
transaction may not be for an Invoice), SalesInvoicePaid (yes/no).

tblSalesOrder = SalesOrderNumber(PK), AccountIndex (FK), DateOfOrder,
CustomerOrderNumber, DeliveryMode

tblTransaction = TransactionNumber (PK), NominalIndex (FK for Nominal tbl),
DateOfPosting, TypeOfPosting, TransCredit, TransDebit, TransactionDetails

The data I would like to view is:

A total for all Invoices for each month for each customer, qryMonthSalesCust
is currently doing this, what I would like is to bring in to the query is a
total of all payments by customer for each month, payments will be
TransCredit and criteria for filtering for only customer payments from the
tblTransaction TypeOfPosting will be 'Customer Payment'.
Then I would like a total for the month, Total Invoices - SumofCredits for
that month.

As I'm typing this I'm guessing Im going to need a new query for the total of
payments, then join the two queries together and use the above sum in an
expression, rather than including the 'SELECT's' in the first
qryMonthSalesCust.

Also is there a way I can bring into the query all the MonthName regardless
of if they have been Invoiced or made a payment for that month. This is only
for 'cosmetic' purposes for my report, as stated in my first post....I would
like to use the query as a sub/sub report for the footer section displaying
monthly oustanding amouunts for the last three months, then anything over
that I would like 'summed' and shown as one total such as overdue.