38 minutes ago, bankruatsteve said:

 

Where is it

How to tell? Water is dripping out where the nut that holds the whole thing in place. It could be leaking from the top gasket, no way to pinpoint. Just have to silicone top and bottom and hope for the best.

19 minutes ago, giddyup said:

How to tell? Water is dripping out where the nut that holds the whole thing in place. It could be leaking from the top gasket, no way to pinpoint. Just have to silicone top and bottom and hope for the best.

 

Leaking from the top gasket will only run into the overflow. It only keeps the basin full what the stopper is in. 

I may be too late, but will offer my thoughts. I have removed three basins from their previous house, and now installed them in my new house. First problem was that I needed new seals, as the existing ones had hardened. Could not find anyone willing to sell me a set of seals. Could order them from Lazada but then not sure of sizes. Eventually bought new bottle traps from Lazada as the easiest way, all with new seals. Also bought new wastes, but then found that I had ordered wastes without overflows and needed overflows. Refurbished the old wastes with pop-ups from the new ones, and repolished the chrome. All good as new. I found that the horizontal pipe that goes through the wall in your case, the top one in your photo of 18 June marked D, is just a bit larger than a 1" PVC pipe. By warming the PVC up a bit you can push the chrome pipe in and get a very good seal. That works for me as I then go down through the floor, or out through the wall.

If you horizontal pipe is still in place, and in reasonable condition, the simplest thing would be to leave it there. Undo the connector at the back, undo your large plastic nut on the underside of the waste, and take the waste and bottle trap away. If you are reusing the waste, clean it up, and make sure that one of the connectors H will fit over the vertical pipe G with the top hat seal A onto the bottom of the waste. If that is all good fit the bowl F into the bottom of the trap E with the large seal C. Slide another connector H over the vertical pipe and screw into the trap E with a tapered seal B. 

You should have been able to slide the old connector and tapered seal off the horizontal pipe. Put your last new connector H and, hopefully, anothe tapered seal B onto the horizontal pipe and screw into your trap.

Good luck.

After a lot of swearing and frustration finally got the bottle trap installed. Liberal use of silicone where the new drain fits into the sink, above and below, plus plenty of thread tape on all the threads. Hopefully be good for another 10 years.