I managed to get this working with your method, thanks for your help 

Just wondering at this point if the hierarchy of the method I am using needs to be adjusted, as changing the class only did not help..

Yes I updated the variable name here is the class

Class BDROWA.Service.BDROWAACK Extends EnsLib.HL7.Service.TCPService
{

Method OnConstructReply(Output pResponse As EnsLib.EDI.Document, pRequest As EnsLib.EDI.Document, ByRef pReplyCode As %String, ByRef pSC As %Status, pEarlyAck As %Boolean) As %Status
{
    Set pResponse=##class(EnsLib.HL7.Message).%New()
    Set pResponse.DocType="2.4:ACK"
    Set MSHStr="MSH|^~\&|EnsembleHL7|ISC|ARiM Server|ROWA|"_$REPLACE($REPLACE($ZDATETIME($HOROLOG,8,1),":",""), " ","")_"||ACK|"_pRequest.GetValueAt("MSH:10")_"|P|2.3"
    Set MSHSeg=##class(EnsLib.HL7.Segment).ImportFromString(MSHStr,.tSC,pRequest.Separators)
    Set MSAStr="MSA|AA|"_pRequest.GetValueAt("MSH:10")
    Set MSASeg=##class(EnsLib.HL7.Segment).ImportFromString(MSAStr,.tSC,pRequest.Separators)
    Set tSC=pResponse.SetSegmentAt(MSHSeg,1)
    Set tSC=pResponse.AppendSegment(MSASeg)

    Quit tSC
}

}

The restart did not help