Well, I've just aged the female bird of my pair - she's 2nd calendar year!
Only 5% of breeding females are this young. And that might explain the infertile egg amongst the clutch.
Also, she lifts her right leg off the perch whilst resting - and one of last year's juveniles, post-fledge, had a gammy right leg - it could be that bird - now mated with her father!!
Known to follow bird waves.