After a cup of coffee sat in the sunshine.... the answer is obvious.....
B4X:
Sub Process_Globals
Private gMap As GoogleMap
Private MyMapType as Object
End Sum
...........
MyMapType = gmap.MAP_TYPE_NORMAL
gMap.MapType = MyMapType
...........
Sub btnFlip_Click 'Flip the map
If MyMapType = gmap.MAP_TYPE_NORMAL Then
MyMapType = gmap.MAP_TYPE_SATELLITE
Else
MyMapType = gmap.MAP_TYPE_NORMAL
End If
gmap.MapType = MyMapType
End Sub