This post is for other users who have had bad experience with the "support" from cute soft that we have had....
Despite 6 emails to support on this matter, I finally worked it out myself.
The weirdly (stupidly) named "EditorWysiwygModeCss" property of the control requires a setting (a css file).
No idea what's in it, but by setting this to "CuteSoft_Client\\cuteEditor.css" I got rid of my double spacing.