I did it!
Using a combination of the previous suggestions, I:
Installed Lumia Storage Check
Set to download maps to SD Card
Downloaded USA map (2.7gb)
In Lumia Storage Check, reset to download maps to phone
Removed SD card
Still in LSC, tapped "maps" button and deleted USA map (deleted map from phone)
Reinserted SD card
Still in LSC, reset to download maps to SD card
Storage on phone gained over 2gb.
Thanks to all for the suggestions,
Craig.