Hi, hoping someone can help as I've reached the end of my knowledge with this particular issue. Essentially I've got a slow draining bathroom sink. It hasn't always been like this and occurred suddenly. The installation is relatively new - only 4 years ago and the property is on the top floor of a 3-story purpose built apartment block.

I've had a fair amount of chemical down the drain to try and unclog it and I've had the trap off and cleaned enough I could drink out of it! I've even had a 15 foot plumbers scope down the drain to try and clear the blockage - which takes it into the main vertical drain.

Interestingly, with the trap off, I'm able to pour water directly down the pipe without issue. However, with the trap back on and when draining from the sink, the water backs up almost immediately suggesting it's only really getting as far as the trap. This suggests an airlock of sorts but I'm really at a loss as I can't think what else the problem could be.

Any suggestions gratefully received :)