Plumbing Issues?

We have some kitchen drain issues in the apartment ladies and gents so let me give you the back story.

We moved into this apartment on 3 April 2014. After two weeks here we noticed a draining issue in our kitchen sink. Basically the water would take forever to go down the drain. If we used the tiniest drop of water it would take 2 minutes to drain. 

After days of this we headed to Woodies DIY store and got ourselves some drain unblock. I was so excited that our drain would now cooperate with us and do what all kitchen sinks do. How disappointed was I when I noticed that after using the 14euro product the drain did not unblock :(

We went back to Woodies another day and picked up a high quality product but still would not budge the blockage, in fact I think it made it worse. The draining was impossibly slow we had to resort to washing our dishes in the bathroom.

As this is a rented property we contacted the letting agents and demanded someone (a plumber) be sent out to us. On Tuesday 3 June a plumber arrived with industry strength drain unblock. As he was pouring it in the smell would literally burn your eyes and nostrils. Both the plumber and I had to stand on the balcony the smell was that bad. 
He left the product in there for 15 minutes and tested, there was still clogging issues. He proceeded to pour boiling hot water from the kettle into the sink. Eventually the block was dislodged and now we have a fully functioning sick. I could have kissed the man I was that happy, until I looked into the sink after he left and saw this.

He left a stain that will not budge, I've tried everything I could think of (bleach being the main ingredient in all products tried). Does anyone have any ideas on how to remove this?

If you know of anything I can try to remove this stain please let me know in the comments below. 

I hope you have a great day. I will see you all in the next blog.