Last week, in response to questioning about spending on Royal Mail, the Home Office minister, Liam Byrne said that the Home Office had spent just £1,132 in 2004-05 and £1,624 on Royal Mail. Now I've double-checked this answer more than once just to be sure that I'm not missing a zero or something because I find it absolutely incredible.
I'm pretty sure that Government will have some sort of Civil Service run internal mail system between departments actually based in Whitehall, and I don't doubt a lot of correspondence these days is also electronic.
However, as a first class stamp costs 30p (prior to pricing changes recently), that suggests that the Home Office sends no more than about 30 letters a day to external recipients and so requiring the use of Royal Mail. Unless of they're using another provider of course.
If there is a civil servant out there who might know why this figure is so low don't stay quiet.