The mean total income of people with self-employment sources of income is shown in 3.10 “Income of individuals with self employment sources, by range of self employment income and source” on the HM Revenue and Customs website at
http://www.hmrc.gov.uk/stats/income_distribution/menu-by-year.htm#310
The tables are based on the Survey of Personal Incomes and are available for 1999-2000 to 2004-05. Information for earlier years can be found in the annual Inland Revenue Statistics publications available in the Library.