Sorry about that Michael <img src="/images/graemlins/frown.gif" alt="" />
Actually, even if the eggs are not fertile, its best to leave them for the full cycle. Sitting on the eggs is not stressfull for the birds and this is a break for them. The hen actually needs this break before she lays more eggs again, or she may wind up calcium depleted.
Its not unusual for a first (or even 2nd clutch) to be infertile. After the 2nd clutch (whether the eggs are fertile or not) please remove the nestbox to give them a break and build up their strength again. After 4 to 6 months, you can give them the nestbox and let them try again.