Addressed comments from first review.

improver/cube_combiner.py

@@ -152,7 +152,7 @@ def _combine_cube_data(self, cube_list):
             iris.cube.Cube
 
         Raises:
-            ValueError: if the operation results in an escalated datatype
+            TypeError: if the operation results in an escalated datatype
         """
         result = cube_list[0].copy()
         for cube in cube_list[1:]:
@@ -195,7 +195,6 @@ def process(
 
         Raises:
             ValueError: If the cube_list contains only one cube.
-            TypeError: If combining data results in float64 data.
         """
         if len(cube_list) < 2:
             msg = "Expecting 2 or more cubes in cube_list"
@@ -236,12 +235,18 @@ def _setup_coords_for_broadcast(self, cube_list):
         match the dimensions, in order, of the first cube in the list
 
         Args:
-            cube_list: (iris.cube.CubeList)
+            cube_list (iris.cube.CubeList or list of iris.cube.Cube)
 
         Returns:
             iris.cube.CubeList
                 Updated version of cube_list
 
+        Raises:
+            CoordinateNotFoundError: if there is no threshold coordinate on the
+                first cube in the list
+            TypeError: if there is a scalar threshold coordinate on any of the
+                later cubes, which would indicate that the cube is only valid for
+                a single threshold and should not be broadcast to all thresholds.
         """
         target_cube = cube_list[0]
         try:
@@ -265,9 +270,6 @@ def _setup_coords_for_broadcast(self, cube_list):
                 )
             else:
                 if found_coord not in cube.dim_coords:
-                    # We don't expect the coord to already exist in a scalar form as
-                    # this would indicate that the broadcast-from cube is only valid
-                    # for a single threshold and therefore should be rejected.
                     msg = "Cannot broadcast to coord threshold as it already exists as an AuxCoord"
                     raise TypeError(msg)
             new_list.append(cube)

improver_tests/cube_combiner/test_CubeMultiplier.py

@@ -118,6 +118,15 @@ def test_multiply_preserves_bounds(self):
         self.assertArrayAlmostEqual(result.data, np.full((2, 3, 3), 0.3))
         self.assertArrayEqual(result.coord("time"), precip_accum.coord("time"))
 
+    def test_exception_for_single_entry_cubelist(self):
+        """Test that the plugin raises an exception if a cubelist containing
+        only one cube is passed in."""
+        plugin = CubeMultiplier()
+        msg = "Expecting 2 or more cubes in cube_list"
+        cubelist = iris.cube.CubeList([self.cube1])
+        with self.assertRaisesRegex(ValueError, msg):
+            plugin.process(cubelist, "new_cube_name")
+
 
 if __name__ == "__main__":
     unittest.main()