Sql Server Procedure Access 2010

I have a problem with a call to a sql server stored procedure which works when called from Access 2010 but not from Access 2007.
This is an Access front end looking at a sql server 2014 backend using DNS-less connections for tables, views, pass-thru queries etc.
This is the stored proc:

CREATE PROCEDURE uspGetStockOnOrder
@OnOrder int output,
@StockID int
AS
set nocount on;
SELECT @OnOrder = dbo.vStock_on_order.OnOrder
FROM dbo.vStock_on_order
WHERE dbo.vStock_on_order.Ord_SppSizeID = @StockID;
RETURN
Go
---------
Within Access, the VBA the code is:
Public Function GetStockOnOrder(Param1 As Long, Param2 As Long) As Long
On Error GoTo ConnectError
Dim objConnection As New ADODB.Connection, objCom As ADODB.Command, strConnect As String, DataSrce As String, RetVal As Integer
DataSrce = DLookup("ServerName", "Master_Central", "User_Loc = '" & Pub_UserLoc & "'") ' different users can have different paths to the server
Set objCom = New ADODB.Command
objConnection.Provider = "sqloledb"
strConnect = "Integrated Security=SSPI; Persistent Security info = false; Data Source = " & DataSrce & "; Initial Catalog=CM_BE"
objConnection.Open strConnect
With objCom
.ActiveConnection = objConnection
.CommandType = adCmdStoredProc
.CommandText = "uspGetStockOnOrder"
.Parameters("@StockID").Value = Param1
.Parameters("@OnOrder").Value = Param2
.Execute
GetStockOnOrder = Nz(CLng(.Parameters("@OnOrder").Value)) ' return value
End With

Exit_ConnectError:
Exit Function
ConnectError:
GetStockOnOrder = -1 ' flag to convey error to calling routine to warn user that query fell over.
errornum = Err.Number
If errornum = -2147467259 Then
MsgBox "The back-end database name or path to the server is not valid." & vbCrLf & " DO NOT CONTINUE!", vbCritical, "DataSense Help"
DoCmd.Close acForm, Pub_FormName
Resume Exit_ConnectError
End If
End Function
--------------------
I have tried using a more complete definition of the parameters in the calling routine:
.Parameters.Append .CreateParameter("@StockID", adInteger, adParamInput, 4)
.Parameters("@StockID").Value = Param1
and even
.Parameters.Append .CreateParameter("@StockID", adInteger, adParamInput, 4, Param1)
But this still bombs out when the .Execute statement is reached.

All my other stored procs (table functions, views etc) and pass-thru queries work fine. It is just when passing parameters, and only when using Access 2007.

I would be grateful if someone could point out what I'm doing wrong (or not doing right).