Error in ebXML SOAP Service

We're moving an Ensemble production from Ensemble 2010 to Ensemble 2017 and in it we have a ebXML SOAP Service.  It extends EnsLib.ebXML.Service.SOAPService and just contains one method with a method keyword of SoapAction = ebXML.

We're receiving a SOAP message with a SOAP action = urn:nhs:names:services:pdsquery/QUPA_IN000011UK02 and this is getting rejected with the error message ERROR #6207: Unexpected SOAPACTION value: urn:nhs:names:services:pdsquery/QUPA_IN000011UK02

We've narrowed down the cause of the failure to the Process method of class %SOAP.WebService.  Where it does Set className=..GetMsgClass(pAction,messageChildlist,.methodName,requestBody)

the methodname is coming back blank. That in turn causes msgclass to get set to "" as $zobjclassmethod(className_"."_methodName,"%New") returns "", which causes the error.

The SOAP message is the same as was previously accepted under 2010. Any ideas what the problem might be?

thanks