Isolate the water, remove the cistern, replace the failed washer, then reassemble and test for leaks.
FL-01Turn off the water and empty the cisternShut the isolation valve on the pipe feeding the cistern, or the main stopcock under the kitchen sink if there's no isolation valve, and check it's fully closed. Flush the toilet to empty most of the water from the cistern, then sponge out the last inch or so pooled at the bottom.
FL-02Disconnect the water supply pipeHold the fill valve steady with one spanner and undo the tap connector nut with another, so you don't twist the valve inside the cistern. Have a bowl or cloth ready to catch the last bit of water in the pipe. Some fill valves have a plastic backnut - loosen it gently, it cracks easily if you force it.
FL-03Remove the cistern from the panFind the two coupling bolts under the cistern, one each side, holding it onto the pan. Undo the nuts evenly, a little at a time on each side, supporting the cistern's weight as it comes free. Lift it straight up and set it on a towel - the base is fragile ceramic and chips easily.
FL-04Inspect and replace the doughnut washerThe rubber or foam doughnut washer seals the join between the cistern outlet and the pan inlet, and a perished, flattened or cracked one is the usual cause of this leak. Wipe both surfaces clean of old sealant and grit before fitting the new washer centred over the pan's inlet hole. Check the coupling bolts too - if they're rusted or the rubber washers on them are crumbling, replace the whole kit.
1 each · Close-coupling doughnut washer (toilet cistern to pan)
FL-05Refit the cistern and coupling boltsLower the cistern back onto the pan, lining up the bolt holes, and check the doughnut washer hasn't shifted out of place. Fit each bolt through the cistern base with its rubber washer under the head, then thread the nut and washer on from underneath. Tighten by hand first, then nip up evenly with a spanner a little at a time on each side - overtightening cracks the ceramic.
FL-06Reconnect the water supply and testRefit the tap connector to the fill valve, hand-tight then a quarter turn more with a spanner while holding the valve steady. Open the isolation valve or stopcock slowly and watch the connection and the coupling bolts underneath for drips as the cistern fills.
FL-07Flush and check thoroughlyFlush the toilet several times and check underneath the cistern-to-pan join with a dry cloth each time - any dampness means the washer isn't seated right or a bolt needs a touch more tightening. Also check the overflow pipe outside isn't dripping. If water still seeps from the join itself after this, the cistern or pan may have a hairline crack, which means a replacement rather than a repair.
FL-08Tidy upDispose of the old washer and bolts. Wipe down the cistern, pan and floor, and dry off any water that pooled during the job. Check the floor and skirting around the base for damp staining if the leak had been running for a while before you spotted it.