Firstly, make sure that the water is Isolated, as the toilet cistern will need to be removed in order to carry out this task. Once you have isolated the water, it will be necessary to flush the toilet so that all the water is removed, giving you access to the unit. You can now wipe out the remaining water with some old cloths etc. Ilford Plumbers can supply and fit new flush units. Disconnect the water supply from the bottom of the tank so that you can undo the bolts that connect the toilet cistern to the pan. If the cistern is secrewed to the wall, then remove the screws. Remove the chain from the handle, and now lift and lay the cistern down on an old towel. Remove the large rubber spud washer and its nut at the bottom of the flush valve using a wrench. Take off the old flush valve and put the new flush valve into the valve hole and visually check the top of the overflow pipe. The overflow should be a minimum of 1″ below the cistern high level mark and the hole where the handle is fixed. Fit the flush valve below the handle lever arm, and fix it to the cistern from below with the spud nut. do the nut up a little more that hand tight with a spud wrench or channel type pliers. The cistern can crack quite easily, so be careful when tightening all fittings. Put the new spud washer over the spud nut, smaller side downwards. Put the cistern down with its front upwards and now fit the correct washers onto the tank bolts and feed into the holes from inside the cistern. Put the brass washers and hex nuts on the bolts from below and tighten them a little more than hand tight. Now place the cistern over the pan and lower it so that the washer sits over the water inlet in the pan and the cistern bolts fit through the holes in the pan flange. Connect the cistern and bowl. If you are not confident you could call an Ilford Plumber to carry this out. Re-connect the water supply at the fill valve inlet. Refit the chain clip on the handle. Connect the refill tube back to the top of the overflow pipe. Test.