I agree with the rest
The check button is absolutely necessary, and can't be too much work.
In case you don't know how (sorry if you know already, but I have no idea about your programming skills) :
An easy way to do it, is putting the answer in an array and comparing that one with the correct solution. That's only a couple of lines of code.
You can use exactly the same to load different sudokus in the same graphical frame : make the sudoku an array with the puzzle, with every position in the array corresponding to a position in the sudoku. Adding a new sudoku is simply adding one array for the puzzle and one for the solution.
I also found the hard one rather easy, but that's mainly because I'm a SUDOKU addict and I solved literally hundreds of them already.
You should also make the "main menu" link a button. Now you have to click right on the text, or it doesn't work.
All in all a nice try, and therefore passed. With a bit extra effort, you can actually make a fun game where you can easily add more puzzles.