We had a new sink and tap installed by a plumber in our kitchen about three months ago. The tap is a lever mixer tap. The tap's fixing to the sink has become quite loose and getting worse. I have looked underneath the sink and cannot see any way of tightening it - there doesn't seem to be a fixing nut or a threaded 'tube'. Because I didn't fix it myself I don't know how it was done. Any suggestions? Do I need any special tools?