When this used to happen in a flat I lived in, I used to play Phil Collins "I can hear thru these Walls..." at full volume - message got through pretty quickly!

However, in your circumstances, I'd be inclined to say that recording 10 mins or so of the fun, then getting up nice n'early (sounds like that wouldn't be a problem) and retrieving the "express checkout" bill from under their door, copying it, returning it, and then sending recording and copy of bill to address at the top of the bill. Then at least you can relax knowing that you've brought some unecessary unpleasantness into the inconsiderate SOB's life - "Who's your daddy now?".

Of course, they may be a couple/married, but do married people really have that much fun?

Ken.