Flats or Apartments

I was changing some of the address data we hold this morning, which had been updated by the post office. This entailed changing a load of "apartments" to "flats". Looking into the address history on the database the last entry for most of them read "address changed as per estate agents" on 14th September 2010. Obviously if you are an estate agent you are going to write "apartment" instead of "flat", because it sounds nicer. I wonder how many of the residents who receive their next mailing addressed to "flat" rather than "apartment" are going to feel cheated. I wonder if you could sue the estate agent for selling you an apartment which turns out later to be a flat? I also wonder who at the Post Office decides if a development is flats or apartments?
Also, is that a UFO behind it?