I have also added some "methods with syntax support" to the SudokuBoard class. This has two improvements in my opinion. Firstly, it allows for statements like this:
to be simply written like:
if (board.cells[l1, l2] == 0)
Secondly, it allows sanity checking of the input, to make sure the board can only contain values that its is supposed to.
if (board[l1, l2] == 0)
Next, I will either, depending on the feedback from my mentor, either continue working on the Sudoku generation code (using the solver I have just written to help), or continue working on the interface, adding error highlighting and a start screen.