Well here's what it says
What evidence can I supply?
You should send any supporting documents to the person you are sponsoring so that they can include them with their visa application. Please do not send any documents direct to the visa section.
There are no hard and fast rules about what documents you should produce when sponsoring a visitor to the UK. You may want to produce evidence of your own immigration status in the UK, 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, celebrating a wedding, a birth or an important anniversary), an invitation giving details of the event, when and where it is to take place, and a letter of sponsorship would be helpful. If the person you are sponsoring is not a close relative or is one of several people invited to the occasion, it would be helpful to know why the applicant was chosen to attend.
If you are offering to provide support and accommodation, or to pay for travel to the UK, you must provide evidence of your ability to do so. The following would be helpful.
A letter of sponsorship giving details of your occupation and salary and how you will provide support and accommodation for the applicant. There is no need to have this letter certified (in other words, signed) before a solicitor or commissioner for oaths to confirm it is genuine.
Evidence of your ability to pay for the trip, such as copies of your last six months' bank or building society statements and salary slips. If you receive public funds, the Entry Clearance Officer will need to consider carefully how long you will be able to support other people on your limited means. You will not be able to get extra public funds to support the applicant.