The number of pensioners paying income tax in 2007-08 can be found in table 2.1 “Number of individual income taxpayers” on the HM Revenue and Customs website.
http://www.hmrc.gov.uk/stats/income_tax/menu.htm
There are an estimated 830,000 pensioners over the age of 80 paying income tax in 2007-08.
The information is based on the Survey of Personal Incomes, of which 2004-05 is the latest available, and projected in line with Budget 2007 assumptions.