I was curious about this as well, and it looks like the SET(M) form will be changing on 22 June 2006. Here's what it says regarding documentary evidence:
We need documentary evidence indicating that you and your partner are still living together as a couple and have done so during the past two years. Ideally, this evidence should indicate joint commitments in your finances, other responsibilities and social activities spread across the past 2 years/ 24 months.
Items of correspondence or other documentary evidence from sources of the kind listed below would be acceptable. These should be divided fairly equally between each of the two years, and be addressed jointly in both your names wherever possible. If you do not have any or enough in your joint names, items addressed to each of you individually may be acceptable, provided they show the same address and you provide roughly the same number of items in each of your names.
The items of evidence should be from at least 5 different official sources. Ideally, a total of 20 items of evidence should be provided. If you are not able to do that, please provide at least 10 items.
Please give an explanation if you cannot provide at least 10 items and/or if the evidence is not in your joint names and/or it is not spread over the 24 month period as indicated. An explanation must also be provided for any periods when you did not live together, as well as evidence in support of the explanation and evidence to show how you maintained contact with each other during this time.
Your application could be delayed or even refused if you do not provide enough evidence of this kind.
• telephone bills or statements
• gas bills or statements
• electricity bills or statements
• water rates bills or statements
• council tax bills or statements
• mortgage statements or agreement
• bank or building society statements/passbooks
• tenancy agreement
• insurance policies/certificates or other correspondence
• loan agreements
• AA, RAC or similar membership
• membership of sports or social clubs
• membership of a religious organisation
• correspondence from government departments or agencies (eg HM Revenue and Customs, Inland Revenue, Department for Work and Pensions) including evidence that you have declared your relationship to the appropriate government bodies.
• correspondence from GP or local health authority (eg: NHS card, correspondence about ante-natal and post-natal treatment,
letter confirming dates of visits to the home address by a midwife, letter confirming registration with a dentist, etc - providing these documents show your home address and the date first registered)
The full application form is available here:
http://www.ind.homeoffice.gov.uk/6353/11406/formsetm0606.pdf