Assistance with small claims court

I need a litter written to send to website Gumtree to obtain contact information of a Private Seller who has mis-sold me an item.
The letter needs to be from a Solicitor on headed paper for Gumtree to provide the details.
I will then be able to take the case to the Small Claims Court.
I can provide all of the information such as advert ID, details of the advertised product vs what was sold etc.