Just a little update incase it helps, i think the problem lies in the WHERE
clause because if the value of the Product code in the combo box is also in
the detail of the last sales order placed then it is showing the record so I
think the MAX function is working, just not selecting the Product code first
prior to MAX..not too sure how to fix it though.. :(

The following may give you what you want.  But I'm not sure.  

SELECT P.[Product Code], P.ProductType, P.Description, P.[Quantity In Stock],
P.[Quantity Description], PO.DateOrdered, POD.Price,
     PO.AccountIndex
FROM tblPurchaseOrder PO INNER JOIN (tblProduct P INNER JOIN
tblPurchaseOrderDetails POD ON P.[Product Code] = POD.ProductCode)
  ON PO.PurchaseOrderNumber = POD.PurchaseOrderNumber

SELECT P.[Purchase Order Number]
FROM tblPurchaseOrder P
WHERE P.Purchase Order
WHERE PO.DateOrdered = (SELECT MAX(TPO.DateOrdered)
FROM tblPurchaseOrder TPO INNER JOIN tblPurchaseOrderDetails TPOD
ON TPO.[Purchase Order Number] = TPOD.[Purchase Order Number]
WHERE TPOD.[Product Code] = [Forms]![frmProductEnquiry]![cboMoveTo])

AND PO.[Product Code] = [Forms]![frmProductEnquiry]![cboMoveTo]

Dear Graeme:

SELECT P.[Product Code], P.ProductType, P.Description, P.[Quantity In
Stock],
   P.[Quantity Description], PO.DateOrdered, POD.Price, PO.AccountIndex
 FROM tblPurchaseOrder PO
   INNER JOIN (tblProduct P
   INNER JOIN tblPurchaseOrderDetails POD
     ON P.[Product Code] = POD.ProductCode)
     ON PO.PurchaseOrderNumber = POD.PurchaseOrderNumber
 WHERE PO.DateOrdered =
   (SELECT MAX(T.DateOrdered)
     FROM tblPurchaseOrder T
 WHERE P.[Product Code] = [Forms]![frmProductEnquiry]![cboMoveTo]);

Hmmm.  Looks vaguely familiar.

May I suggest this:

Temporarily remove the last line, so it returns all [Product Code]s.  You
get some results, right?

Assuming you do, then it is this filtering that is at fault.  Choose one
specific [Product Code] produced by this query and Copy it.  Paste it into
your query in a new last line reading:

 WHERE P.[Product Code] = "XXXXXXXX";

where the X's are what you paste.  Omit the double quotes around the pasted
value if it is not a text value.  Run this.  Does it produce exactly that
product code?

Make sure the form is open, and that a product code has been entered into
the cboMoveTo control.  Change the first line of the query to be:

SELECT [Forms]![frmProductEnquiry]![cboMoveTo] AS cboValue,
   P.[Product Code], P.ProductType, P.Description, P.[Quantity In Stock],

This will now show the exact same thing as before, but with a new first
column, being what was entered into the combo box.  Does the cboValue column
now exactly match the [Product Code] column?

Please let me know what you learned from this.  Perhaps we can soon discover
where the error is.

Tom Ellison