I need a solicitor to write a letter

My partner’s brother has accused me of causing damage to his property, (attempting to cause a flood by damaging a toilet and cutting an external TV supply cable). He says he has video proof of me cutting the cable! He has now told his mother (and probably others) that I have caused the damage. I need a letter or some other device to compel him to stop him making unfounded accusations and apologise or provide the proof that he says is in his possession. Can you advise of the cost etc