The story's called Now or Never (http://www.whofic.com/viewstory.php?sid=22323). I recommend her other stories as well. She hasn't written in a year though, which I just think is a shame. As for the UST between the Doctor and Donna, I never got it from watching S4 (although I got a whiff of it during the cafe scene in EoT) but I know a lot of other people saw it. It's a matter of interpretation.

Also, try this fic rec (http://beagleinspace.livejournal.com/6083.html). Beagleinspace has divided the fics into categories reflecting the Doctor and Donna's relationship.