Fixing Basin Waste Pop Up
There are two things that seem to go wrong with pop up basin waste plugs, either the clicker stops working or the O ring washer fails.
Clicker
The clicker can stay stuck down, refuse to stay down or come apart.
If it sticks down it can possibly be pulled back up with a drain plunger, otherwise you will need to take off the U bend and poke it back up with a long screwdriver. To take it out, unscrew the plug from the top and use a small adjustable spanner to remove the clicker. It will be choked up with sludge and hair, clean it out and put some petroleum jelly or WD40 on it for lubrication. It should then work again.
If it refuses to stay down, the internal grooves for the mechanism are probably worn. You will need to get a new one. See above for how to take it out.
If it comes out completely, the small retaining screw has probably fallen out. You will need to take off the U bend and attempt to rescue any bits that have dropped down. If you can find all the bits, it is possible to reassemble it, you need a pair of long nosed pliers, or something similar to get the piece of bent wire back in it’s hole at the bottom end. Otherwise, get a new one.
Replacement clickers can be found on eBay, make sure you get the right type, there’s more than one.
Washer
I have found it almost impossible to get the correct replacement O ring type washer for the pop up plug. The only ones I found that might be correct cost more than the whole basin waste assembly complete. I ended up with some 38mm O rings from Amazon that weren’t quite fat enough, so I fitted two of them, which seems to work.