Accused of theft of a cat!

[Guest]

Hi all. I have been accused of stealing someones cat. A friend sent me a screenshot of a Facebook post done by a friend of his concerning someone stealing their cat. This mans partner then commented on his post, naming me and saying where i lived and accusing me of theft of the cat. Several unpleasnt comments followed by their friends about my character. My children attend the same school as the accusers and i am getting comments in the school playground form people i dont know that i am a thief etc etc. I also recieved messages form the man who made the post demanding his cat back. I have told him in no uncertain terms that i dont have the bloody cat. Ive also told him i will contact a solicitor for defamation. He has since deleted the post and comments but i have the screenshot. Ive logged a complaint with facebook already. My question is: Does anyone know how much it would cost me to send a solicitors letter threatening them to stop with their lies and accusations and can my screenshot be used as proof?