Yes, "board", the same nomenclature used in the solution description!

I've just reviewed the code again and see that I was indeed incorrect - couldn't see the wood for the trees.
The data storage is what threw me. I assumed that the boards are stored as data arrays, so the first array is the entire board in the top left corner, rather than the data being a literal view of the entire sudoku board, as in the first array is the entire top row.

I'm not sure if this data storage is a good idea or not (I'm inclined to think not!) but it may have been intentional on part of the author to add another layer of data translation into the kata.

As such, I'll remove the issue flag and fix my code ;)