Nope, I'm not going mad ... (phew)

```
>>> cubes = iris.load('NAMEIII_field.txt', callback=callback)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/net/home/h05/itwl/projects/git/iris/lib/iris/__init__.py", line 198, in load
    return _load_collection(uris, constraints, callback).merged().cubes()
  File "/net/home/h05/itwl/projects/git/iris/lib/iris/__init__.py", line 168, in _load_collection
    result = iris.cube._CubeFilterCollection.from_cubes(cubes, constraints)
  File "/net/home/h05/itwl/projects/git/iris/lib/iris/cube.py", line 135, in from_cubes
    for cube in cubes:
  File "/net/home/h05/itwl/projects/git/iris/lib/iris/__init__.py", line 155, in _generate_cubes
    for cube in iris.io.load_files(part_names, callback):
  File "/net/home/h05/itwl/projects/git/iris/lib/iris/io/__init__.py", line 177, in load_files
    for cube in handling_format_spec.handler(fnames, callback):
  File "/net/home/h05/itwl/projects/git/iris/lib/iris/fileformats/name.py", line 80, in load_cubes
    [header, field_headings,
NameError: global name 'header' is not defined
```