if you get a tap that has loosened up because of a split top hat and you cant undo the water connection or cant get to it easily (i have had this happen before) . instead of undoing the water connection , jest unscrew the backnut a bit down the tap thread . then cut a 1 1/2" (ish) square piece of ply and drill a central 20mm hole in it, then cut from side to the hole as to create a kind of C piece of ply that can be slid easily inbetween the bottom of the sink and he backnut , then tighten in back again , this acts as a tophat without the need to dissconnect the tap if you cant turn the water off or something!