It is working pretty well actually. I'm manually drawing the grid squares then moving an edit control to the appropriate place on mouseclick on a value, setting the text to the same as the value and bingo. When the user clicks another value, it stores the edit text in the current value then moves the edit again. Works pretty nicely.
Next up, following enmaniac and Evil Steve's helpful advice in the forums, I need to subclass the edit control so I can make Up, Down and Tab move the edit box and Enter close the form.