Turn off water. Disconnect water supply and overflow pipe (if fitted) from bottom of cistern. Flush toilet to remove as much as possible from the cistern, then disconnect metal link from the siphon to the lever arm. Remove any screws holding cistern to the wall, then undo the 2 bolts under back of pan, holding the cistern in place. (If totally corroded, you'll have to cut them off with a Junior Hacksaw and replace.) Taking care, cistern should now lift off the pan. Discard remaining water in cistern, and place cistern of a suitable soft surface that wont damage cistern and wont matter if damaged, piece of cardboard is ideal.
Remove Doughnut washer from end of siphon tail, and discard. Undo and remove large retaining nut, remove metal bracket (if fitted) and again, if too corroded, replace. Siphon should now be free, remove and either, replace the washer is that is your preference, or fit a new siphon. (Check height before purchasing, they do vary.) If replacing Siphon, look at the type which come apart for maintenance, the Dudley Turbo is one such example. Means you can replace the washer etc, without having to dismantle the Toilet!
Make sure area around siphon tail hole in cistern is clean, fit/refit siphon with new washer on the inside, replace bracket (if fitted), and ensure holes line up at 3 and 9 o'clock, before tightening nut to secure siphon/bracket in place. Fit new Doughnut Washer to tail of siphon, replace bolts in bracket, and carefully offer cistern back onto the pan, making sure it cannot fall, whilst replacing washers and nuts onto the securing bolts. Retighten nuts in unison so cistern is pulled evenly down onto pan. Reconnect water inlet, extreme caution with the plastic ballvalve, very easy to cross thread plastic, and overflow, restore supply, checking for leaks. Reconnect link to flush lever, and test.
