@tig said:
When you export to fbx [or several other formats like obj] there's an 'Options' button bottom-right of the dialog... that has "export-double-sided-faces" as an option to 'tick' - isn't that what you really want to do ???
facepalm I remember seeing the options button once or twice but each time I focused on something else. Worked perfectly, thank you and sorry for the trouble