Bad smell in bathroom. Please help!

[Gymohithoughtyousaidgin]

So I don't know if anyone has had the same issue before. We have a seperate toilet (we are in a 1st floor apartment, building is 15 years old), and all of a sudden we had an awful smell coming from the bathroom sink where the water drains. I have had a plumber come out and they have taken the kind of bendy pipe bit out to clean and they have cleaned as far down as they can and it has made absolutely no difference. I have used endless bottles of bleach, vinegar and every kind of sink and drain unblocker you can think of. Plumber has said he's got no idea and can't do anything else.
Does anyone have any idea what it could be? The smell is somewhere between rotting cheese, mouldy seashells and Death 😂 we can't use the room because of it. Please can someone give us any advice?

We had this. It turned out there was an invisible leak through the tiled floor and the floor joists were rotting!

I'd also go for leak under the floor boards....

Has anyone else in the block got the same issue? There may be a problem with the drains outside the building and it's just that the bathroom is the place where the smell is escaping.

I was also coming on to say a leek under the floorboards. In our case we also had dead and decomposing slugs ( neighbour had put slug pellets down shared drain for ground floor WC's)

*leak

Wet towel or cloth festering somewhere under the sink is a common cause of such smells.

If your sink has a functioning bottle or p trap then the water should stop bad smells coming back up. Is the smell constant or occasional? If occasional then shine a torch down the waste hole when the smell occurs and see if the trap is full of water.

If a constant smell then I would expect it to be closer than your trap. Does your sink have an overflow hole? That could be blocked and stinky, pipe cleaners or a narrow bottle brush to break it up and then get bleach back there. . If you have a pop style plug then unscrew it and check underneath, they can get a foul build up of scum on them.

If all is fine I would suspect that the problem is not actually coming from the waste.

We sometimes had this with the drains and I would pour neat Zoflora down the plughole, shower drain and capful in the loo then leave and flush it with a kettle full of boiling water.

If the smell is constant and a plumber isn't even sure - I'm tempted to think you need to call another plumber because there's something causing it. Possibly a blocked drain or section of pipe further along with a dead festering bird stuck in it.

@Sundancerintherain

I was also coming on to say a leek under the floorboards. In our case we also had dead and decomposing slugs ( neighbour had put slug pellets down shared drain for ground floor WC's)

Ah, you live in Wales.............

Thank you all. Will get the land lord to look into the possibility of the leak under the floorboards. Il check with the neighbours too to see if anyone else is having the same issue. The ptrap is the bendy bit isn't it? It had a build up of crap that the plumber cleaned but the smell is still just as bad and its constant it doesn't go at all! Thank you for all the suggestions!!!

Yes the p trap is the bendy bit. The design means that water sits in the bend and acts as a seal to stop sewer gasses coming back up the pipe.

Could be a dead rat in the drain?