Apartment on Rent review: Fraud by landlord

Hi Team,
http://www.gumtree.com/p/flats-houses/looking-for-indian-to-share-well-furnished-3bedroom-house/1050693263
http://www.gumtree.com/p/flats-houses/furnished-doubled-bedroom-available-for-360-including-all-bills/1049576017

Above ad publisher is a fraud person, he fraud me on last dec, 2013.
He is subletting his rental apartment and misbahave with the tenets after that.
I had sent following legal notice to him, but he returned my notice saying no one is available in his room.
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Dear Navin Tyagi,
As per your add on ‘Gumtree’ for rent on 20th Oct 2013, I came to you double bedroom flat on 23th Oct 2013.
As you were not able to provide me rent agreement, I gave you notice to leave your flat on 1st Dec 13 for leaving your room on 14th Dec 13.
Now my one month rent advance money (£450) and rent of half month (£225) as total of £675 is due on you.
You are subletting that flat and not providing rent agreement to tenants.
I requested you many times but you are not returning my money and you blocked my number to receive my calls.
As per the legal process, I am sending this prior notice to you.
Please therefore refund my deposited money £675 within seven days from this date.

Yours faithfully,
Tarkesh Pandey
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Request to your admin/Legal HR team to take serious action against this person .
He used to publish ad on your website with different names i.e. Rohan, Ranna, Taj.
His original name is NAVIN TYAGI and his number is [protected], [protected] .
Many Thanks,
Tarkesh Pandey
+44 [protected]