Ok just changed the code property to :

Function ReturnXML(ByVal MultiValueList As Object, ByVal Root As String, ByVal Node As String, ByVal Element As String) As String

'**************************************************************************

' Returns an XML string by using the specified values.

' Parameters:

' MultiValueList - a multi value list from SSRS

' Root, Node, Element - String to use in building the XML string

'**************************************************************************

Dim ReturnString = ""

Dim sParamItem As Object

ReturnString = " "

For Each sParamItem In MultiValueList

ReturnString &= " "

Next

ReturnString &= " "

Return (ReturnString)

End Function

now my error is 'parameter '@C' contains an error: [] Name ReturnXML is not declared.

My dataset Parameter code is: =ReturnXML(Parameters!C.Value,"C","Customer","Customer_Number") - C being the name of the multivalue parameter which is defined as string the the parameter details.