The average income of self-employed taxpayers in Herefordshire can be found in table 3.14 “Income and tax by borough and district or unitary authority” on the HM Revenue and Customs website at
http://www.hmrc.gov.uk/stats/income_distribution/menuby-year.htm#314
These tables are only available for 2003-04 and 2004-05 and are based on the Survey of Personal Incomes.