Bathroom Wash Basin Leaking and Water in Sink Cavity All aspects of plumbing questions and answers, help, tips and information

[spacepods]

Long post but will give as much as I can (previous post got lost somewhere)...

History
I had a problem of flooding in the bathroom, water in kitchen below, after turning on the water supply (had tried fitting new monobloc mixer in kitchen).
The wash basin was full to the overflow (wasn't overflowing the wash basin). and the water was dirty and full of bits.
This was possibly because the waste pipe was blocked. I am pretty (95%) sure that the wash basin tap (mixer) wasn't open when I turned the water supply on.
I turned the supply off again and got the plunger out and unblocked the sink/waste pipe. I added some sin/drain blocker liquid. This appeared to rectify the issue and the water now goes down the plug hole easily.

Now, I believe that the water running into the bathroom had come from the one cast hole(s). I understand that this is probably caused by a crack in the basin and therefore the basin needs replacing.

My question...
There is a drip from the cast hole now, but I am unsure why. It's extremely slow (a cupful every 24 hours I guess). It drips when the tap is closed (no water going into the basin).
The flexi hoses to the tap behind the pedestal and all other fittings there appear to be dry.

Is there anyone that could explain this to me and where the water in the cavity (and causing the drip) can be coming from?

[spacepods]

A little more info...

I have taken a look closer look at this and it appears that a blocked overflow/waste can cause the cavity to fill.

Through the overflow hole, to the right there's a ceramic wall. This isn't the case to the left. I can push my finger in without resistance as far as it will go.

Therefore I think that the water has filled up the overflow and starting filling up the cavity. When the cavity filled, there's only one way the water can come out - through the cast holes.

See attached image for how I think the wash basin is designed and what has happened. Is my understanding correct?

I have siphoned off some of the water from the cavity but I still get moisture build up and dripping. Could this be from condensation perhaps?