I recently fitted a replacement Dudley flush syphon, a few days later noticed water dripping from the left hand wing nut thread. I emptied cistern, dried all round the flush down pipe, noticed that black was coming off from the internal washer, so maybe perishing after 27 yrs! Applied Fernox leak sealer all round the washer and is holding at the moment but probably have to take the cistern off at some point to replace washer, doughnut, fixing plate and securing screws. On a YouTube video on the cistern being worked on there was a thin washer between the plate and the cistern. Is this essential, I can't see one on other videos? Also some say to apply plumbers mait to the doughnut, is this necessary and if so do you put it on top, bottom or both?