start at the BARB website www.barb.co.uk and click on viewing summary on the left. quoting from this page- "Choose Weekly Viewing or Monthly Viewing from the menu at the top of the page and select the report you wish to view.
You will be presented immediately with the latest month's figures and you may access backdata from the drop-down menus on the report."......"The IPA provides quarterly data through IPA Trends in Television - www.ipa.co.uk"