I'd be interested to see if the tap turns on etc if somebody is actually in the bathroom watching it, or if what is being experienced is just an "echo" - you hear the sounds etc, but the physical components don't actually move.
There are several ways of getting a resident gribbly to move on, some "harsher" than others... The first, and the one I would recommend for a deceased family member, is to simply ask. When the activity is occuring, simply ask for it to stop, and for the person responsible to move on.
If this doesn't work, I will share other methods by PM. There is one in particular of my own devising that is very effective.
As to why these sort of things happen more often at night... I think that ghostly activity happens all the time, but we are more able to perceive it at night for several reasons: we, as humans, seem to have an innate fear of the dark; because of this and the lack of other sensual stimuli (such as a lack of light to see by, and things being quieter at night) our sense are "enhanced" to a level that makes detecting subtle things easier. There's also a theory about changes in the Earth's atmosphere at night when it is not exposed to the Sun's light/radiation, but I don't know the science enough to make any assumptions.
And Lord Lucifer - if that comment was loosely directed at me, I am open minded to all sorts of explanations. I just like the wonder of things, and I don't want to see science take all the magic from the world. If this was a "Scientific questions answered here..." thread, I would be focusing on scientific explanations, but since it's not...