So, what if Ace thinks the Doctor is trying to get through to her, though it would start subtle (I have some ideas) And then it gets weird enough she finally contacts Sarah Jane.   A cryptic message is sent, and off they go a-hunting.  The point of driving Ace nuts is that if they started with just a "The Doctor Needs You" Ace would be suspicious.  It has to be enough that when she thinks she has a real lead, she'll take it without thinking too much about it.  I'd be happy to NPC sneaking up on Ace's brain for a bit.

Maybe, if you want, you could do the same for Sarah and we could torment each other for a while.  LOL I'm imagining Ace getting caught while Sarah has the wherewithal to know it's a trap. She goes to investigate and finds Ace there. Though, I'm not sure how it would work out exactly. 

I"m seeing a sort of haunted house thing.

And I'm thinking that what "they"  want is to try to get secrets about the TARDIS (and the Doctor, maybe) 

I hate to come right up at odds with you, but if they're looking for information about the TARDIS, I assume they'd know very little about it.  Also, they're not hoping for her to just sort of start talking about it, what they want is Ace locked in somewhere while they try to get direct information from her.  Sarah can come break her out, then they're on the run and trying to solve who is after them at the same time.  I do already have some ideas about how to torment you.  Err, I mean forward the story along.