A while ago, I thought I explained this to you. I'll try again.
The Family Tree Maker program, create a File. NOT a Tree. Within that file, there WILL be Many Trees. Each tree depends on who you are looking at, at the time.
If you are on yourself. You have an Ancestor Tree AND a Descendant Tree.
If you go to your parents, there are Ancestry and Descendant Trees for both of them. If you have siblings, your Parents Descendant TREEs will be Different then yours.
So, there WILL BE Many Trees in your Family Tree Maker file.
See, this time you gave me what I asked for:
You said "SPOUSE as Alexander James Macdonald followed by Natural - Ongoing"
So, If you are in the People Workspace (should be the orange button at the top of the screen), the Person Tab, on the left, and have Hi-Lighted Janet Scott, the Relationship Button (upper right, or third button) the above relationship is correct.
However, you continued:
"latest Relationship again gives the SPOUSE as Alexander James Macdonald but followed this time by Spouse - None."
Now, I am confused. Are you seeing TWO Spouse Relationships between Janet and Alexander?
If that is the case, that may be why you have having a problem.
Which one has a Preferred Check Mark? There should be a Preferred column on the right of the screen.
Please click on the Family Tab, from that screen. Look under Janet for the Spouse ICON. Does it, by any chance, had a  Next to it?