And apply! If they don't like the sound of you they won't respond. I applied for a Second in department post after two years of teaching and never heard back from them. I applied for a Head of department job after four years of teaching, thinking it was way out of my league, and got it and am perfectly fitted for it. You could also offer to shadow your current HOD and see what is involved - even if you do decide to wait for a bit longer it will give you some excellent pointers for your application.