Also, have we covered all of the cases? There are lots of different grid types:

```
>>> import iris.fileformats.ff_xrefs as f
>>> set(f.STASH_GRID.values())
set([1, 2, 3, 11, 14, 18, 19, 21, 22, 23, 25, 26, 27, 28, 29, 36, 37, 41, 42, 44, 60, 62])
```

It'd be nice to document any cases which aren't covered.