When I found the house I know own, it had such a STRONG pull on me.. I KNEW it would be mine.... the house itself had a spirit that "needed" me... but it wasn't until after I moved in that other things became more obvious... thankfully nothing malicious, all neutral things.
There's at least 2 spirits other than the house's own spirit here... there's a young child.. he mostly hangs out in the basement area... there's a part of the basement that used to be for coal storage, it's set apart from the main basement by a wall and door and the times we've gone beyond that threshold were VERY uncomfortable times. A friend of mine refuses to go through at all (she's very sensitive to spirits... )... there's also an older spirit, if that makes sense though the child, we feel, has been here, pre-house, more with the land.
When I moved in I just did a general cleaning... the house came with it's own brooms (huzzah!) and I broomed through the rooms, sweeping out the old before setting in the new (us)... I also censed with incense. After an odd ocurance shortly after I moved in (which had more to do with something moving swiftly through the house and wasn't at all very nice) we salted all opening... all doors, windows and such... I also added pieces of hematite in rooms that needed it most.. living room, my room and my son's room... (hematite to soak up and disperse negativity)...
We still have occurances here but nothing like we did. The house still wants us here... and while they make their presences known once in a while we're all one big happy lot.
I think it's an odd place that doesn't have some kind of activity. I grew up in a house that had a not so nice feeling in a closet that was a part of my bedroom... later I found out my sister (who also had that bed room years before) had similar feelings... I only had evidence of the closet door opening and such... she had mud on her mirror one morning....
Good luck with the new flat... have you tried using a pendulum there?
Also another thing I've heard some folks talk about is requesting spirits leave before you occupy a place... likewise I've also heard folks invite spirits from one domicile to the next... guess it all depends on what kind of 'company' you want. :)