Could be one of a couple of things could need a new rubber washer as late at night less people use water so loal water pressure rises hence if washer is worn water may pass under pressure ,or it could be the cone shaped part that the washer sits against (the proper name for this escapes me at min)that may have a hairline crack if its getting on in age,or it could be the float adjuster has worked loose and is causing over flow to run allowing water level to drop constantly usualy a marking on back of cistern for correct water level hope this helps
cpd services, September 2008
