I don't know if anyone has already posted this ...

For people writing to government departments about the numbers those departments publish.

The Central Office of Information "Better practice guidance for Government contact centres" document is available for download from the main COI website. See:

http://www.coi.gov.uk/aboutcoi.php?page=58

You have to register your name & address before it lets you download it.

Unfortunately it only says 0870/0871 are "not recommended" rather than disallowed and recommends 0845 because they get less prank calls than 0800 - although it does acknowledge that 0845 costs more to call than geographic numbers.

-- Sorry - I see this link was already posted on Jun 11th - I didn't search back far enough before posting it.