After reading the suggestions here I thought I'd go off and give them a go and report back on any progress.
I think MaggeRoseLee's statement here -
I don't think it's separation anxiety.
I think he's worried that the person moving off is going into GREAT DANGER and is only safe when everyone is together. Or that the other person doesn't realize they are leaving the pack! So all the noise is communicating 'I'm over here!!!'
might be most pertinent to our situation.
A few days ago we took him for a walk around a busy/urban environment and my SO ducked into one of the stores while Baron was taking in all the sights/smells/sounds. I had the leash, so me and Baron just kept walking. After a few metres, Baron turned around to see that SO was gone, and seemed to want to go and find him but I just kept moving at a semi-fast pace. There was lots of turning his head, but he seemed to settle and we were able to continue our walk as if nothing happened. No straining, whining, crying, barking or anything had happened even once. I was totally shocked. When we walked in the direction that my SO had slipped off, he seemed to get back into pulling to find him, but still quite manageable with no whining or barking. It was only when I saw my SO on the other side of the street, looking for us, that I called out his name. When Baron heard me call his name his ears pricked up and he starting pacing on the end of his leash. I had to call out again, since my SO didn't hear me the first time and when I did this Baron let out a whine-bark. I suspect he was doing exactly what MaggieRoseLee said, and was trying to let my SO know where we were. The only time he made any fuss was when I had called out my SO's name (perhaps Baron thought I was distressed?) so I am incredibly happy with his progress and will continue to work on this.
He also whines if one of us leaves the car, but I was able to get him quiet after just one 'enough!'.
Thanks to everyone for your insight and suggestions
and I hope you see some progress soon too Springbrz