Thanks Guys,
UnborN, you are right re the function node reacting to every message, but I was hoping the trigger sending one message per minute would mean the function node would do one message per minute.
I'll look more at the context you mentioned.
jbudd, you are right, with the inject node name being what shows, but the node was also configured to 1 minute.
the msg-speed is a node that just counts the rate of messages being received. This is what it looks like after running all night:-

60/hour indicates it's receiving 1 per minute.
I'll re-jig the flow and post shortly.
thanks
