I have used the scripts set out here: Solved: API Python Workspace and Model IDs - Anaplan Community to successfully extract a basic model and workspace list.  Thanks to @abhay.kanik for the good start.
 
I have added the ?modelDetails=true switch to the Model JSON so the API send more details about models.  The JSON then contains the following information:
| 'id' | 
|  'name' | 
|  'activeState' | 
|  'lastSavedSerialNumber' | 
|  'lastModifiedByUserGuid' | 
|  'memoryUsage' | 
|  'currentWorkspaceId' | 
|  'currentWorkspaceName' | 
|  'modelUrl' | 
|  'categoryValues' | 
|  'isoCreationDate' | 
|  'lastModified' | 
 
I can refer to and extract lastModified and ActiveState with no issues.  However, when I try to include the model 'memoryUsage' in my export Python returns a "KeyError".
 
The code I have modified is:
for mdl in model_json['models']:
model_writer.writerow(['{}'.format(mdl['id']), '{}'.format(mdl['name']), '{}'.format(mdl['activeState']), '{}'.format(mdl['memoryUsage']), '{}'.format(mdl['lastModified']), '{}'.format(wsp['id']),'{}'.format(wsp['name'])])
 
If I remove the memory usage request the script runs fine and gives me the other results.
 
Any suggestions on what I may be doing wrong here?