@Markblackburn86
No, it is not that easy!
The solution here refers only to a 'small' square, e.g. 9 Cells = 1 Dropdown
In essence for a 'real' working solution you would need 81 ! Dropdowns, namely 1 for each of the 81 Cells of the 'big' square. This is because you have to eliminate numbers also if the are already used in their row and column of the big square. Working that out will be a real challenge.