Place a basin under the trap to catch any loose water. Undo the U-bend by hand and take it off.The trap should now slide away. Save the washers and nuts by leaving them on the tubes. Undo the ball-and-pivot rod in the pop-up body and take out the ball. Compression of the spring clip on the clevis will allow withdrawal of pivot rod from the clevis. Lambeth Plumbers can provide expert help. Take out the pop-up stopper. Undo and remove the lock nut on the stopper body from below. Push the body through the hole freeing up the flange from the basin and remove flange and stopper. Put the locknut, flat washer and rubber gasket down the new body (bevel upwards). Put some teflon tape onto the top of the body. Use plumbers putty on the flange, and place underneath the drain flange. Put the pivot rod opening facing directly back toward the middle of the sink and pull the body straight down to seat the flange.Thread the locknut/washer assembly up and fully tighten with a wrench. Do not break the putty seal on the flange. Place the new unit in the drain hole so the bottom hole of its post is closest to the back of the sink. Put the nylon washer into the opening in the back of the pop-up body with the bevel facing back. A Lambeth Plumber has the skills and equipment to complete the job. Sandwich a hole in the clevis with the spring clip and thread the long end of the pivot rod through the clip and clevis. Put the ball end of the pivot rod into the body opening and into the hole in the stopper stem. Screw the cap on to the body over the ball. Place the cap behind the ball on the pivot rod. Wedge a hole in the clevis with the spring clip and thread the long end of the pivot rod through the clip and clevis. Place the ball end of the pivot rod into the pop-up body opening and into the hole in the stopper stem. Screw the cap on to the body over the ball.