BUT ... as it stands the user can click on an already checked button and it turns off.
The behaviour of a "check button set" is that there is always one button active, the above should do nothing to the already checked button.
But I think a little trickier than that.
But I didn't explain fully, I'm looking for a button set where
1. One button is always on.
2. Initially one button is on.
Can't remember what this is called in interface design ... the user selects one of N options.