As already stated the HO doesn't ask for such letter, fucus on gathering the documents they ask for, having said that, i found this on another forum (Apologies if this isn't allowed)
To whom it may concern;
I, NAME, am NAME RELATION (Brother, Sister, Friend, Coworker, Etc.) and have known him for XX years. I am writing this letter to attest to the validity of the Marriage between NAME and NAME. I have been a witness to their relationship since they (Dated, were engaged, Got Married, While they were married) and they are truly in love and have no deviant reasons for their marriage other than them wanting to spend their lives together.
Not only have they joined in celebrating (Religious holidays, birthdays, weddings, anniversary’s, family activities, etc.), I attended their (engagement, Marriage, Baby Shower, Etc.) They are a happy and loving couple and I was over joyed to hear the announcement that they are currently expecting their first child on August 7th 2012.
I, NAME, declare that all statements made in this document are true and swear to its legality.
Regards,
(Signature) Date
NAME
Contact number
Email
