I've just done my first BOSS scan since installing Dawnguard. I had several warnings, one being UFO. I'm running UFO 1.2h. Other than not getting a "You know" option for Serana it seems pretty much okay, but I'm still getting this:

• UFO - Ultimate Follower Overhaul.esp Active
  
  • Warning: UFO - Dawnguard AddOn.esp required to work properly with Dawnguard (Official Bethesda DLC)
    
  • Note: For safe install need dismiss all your followers, save, delete all included mods, load again, save and install UFO. For safe uninstall or update see special instructions on http://skyrim.nexusmods.com/mods/14037.

I've checked and UFO 1.5h doesn't say anything about being incompatible with Dawnguard. Any clues as to what I should do?

Well, uh... the obvious thing that comes to mind is that you should update the mod and install the Dawnguard UFO addon?

^ what he said

I've just had a more thorough check of that word wall on the UFO page and the Serana problem isn't the thing the addon is fixing. Trolls of all things! God, I hope this doesn't mean I have to go through the hullubullu of updating the whole of UFO...yet again!

I'm pretty sure the Dawnguard UFO addon is supposed to add UFO functions to Dawnguard followers, like... Serana. I'm not sure if you HAVE to update the whole mod, you might be fine with just installing the DG addon. Be sure to dismiss Serana first to avoid any mess-ups. I'd update the whole mod with all that clean save procedure if I were you though, it's like 5 minutes of work at most anyway.