Love Hazel, but not for a blue eyed blond baby. Love Annabel too. I seem to be drawn to 3 syllable girls names and love it along with names like Vanessa, Melissa, Elana. I don't like Luke though (sorry, if already decided). Sounds weak and a bit drippy.
Both of your children have names that end in -er. I wonder whether this is a subtle trend you would like to continue with this baby. If so, you might like:
Amber Aster Esther Jennifer Harper Summer
I fear most of those are probably a little similar to your children's names so, for that reason, you might want to look further afield.
I would choose Clara from your list with Annabel a close second. The other three are very pretty but I think are either too similar, or too different, to Alexander and Heather.
I am not sure whether you are looking for any additional suggestions but I think these names sit well alongside Alexander and Heather: Lydia, Christina, Elizabeth, Charlotte, Emily, Catherine, Rebecca, Abigail, Iona, Ailsa, Bridget, Orla.
Nothing else suggested trumps the ones on the list but great to hear all the opinions. Looks like Clara is winning. The problem is ds would have been Clara had he been a dd so I wonder if the magic has waned a bit caused we've talked about it for so long iykwim