from:
http://www.ukvisas.gov.uk/servlet/Front ... 6977150088
What evidence can I supply?
There are no hard and fast rules about what documentation you should produce when sponsoring a visitor to the United Kingdom. You may wish to supply evidence of your own immigration status in the United Kingdom, and there are some other things you can do, particularly if the visit is for a special reason:
* if the visit is for a holiday and you are a relative or a friend, a letter of sponsorship explaining your relationship to the applicant and why the visit is taking place at this particular time may be helpful.
* if the visit is for a special family occasion (for example a wedding, a birth celebration or an important anniversary) an invitation detailing the event, when and where it is to take place and a letter of sponsorship would be helpful. If your visitor is not a close relative, or is perhaps one of several people invited, it would be helpful to know why you chose to invite them.