I have now "seen the light" about how this has been designed, amended the outbound mqtt message from the controller to request ACK:
and this is working and sending back an echo message:
mygateway1-in/100/2/1/1/3 100 mygateway1-out/100/2/1/1/3 100
So I can now build a rule round this in openhab to handle errors. I suppose I should have thought this through before pestering the forum but then again that is what it's for! Thanks again.