I have had a new wash hand basin plumbed in but there is a leak coming from under the nutbolt that is actually holding the basin waste to the to the basin. the plumber has changed the washer, the bolt and used copious amounts of silicon and plumbers putty to no effect. After the tap is turned on, it takes a minute or so for the water to start leaking down through. anybody any ideas as this is driving me insane! 
