I'm referencing from the SET(O) application form itself, on page 74 "Section 12 - Documents" then subsection "12A All applicants", then on page 75 "Your Finances" the following paragraphs:
For all applicants except those applying in the bereaved partner category.
Evidence of your finances. If you have to complete section 5, bank statements, building society savings book(s), pay slips or other formal documents as evidence of your ability to maintain and accommodate yourself and any dependants without recourse to public funds. (See Note 4).
Note 4: We do not accept cashpoint statements as evidence of finances. Online statements can be accepted if accompanied by a supporting letter from the bank or building society, on company headed paper, confirming the statement provided is authentic or if they bear the official stamp of the bank or building society on every page. If you claim that a relative or friend is providing you with financial support (see question 5.4), in addition to evidence of your own finances, you must provide bank statements or other documents of the kind described above as evidence of their financial resources. The documents showing the finances available to you and to any person supporting you should cover at least the last three months.