What is Britain's debt?

PM Brown in Parliament said Britain's debt was 37.5 per cent according to IMF figures for 2008 and then quoted several other developed countries all with higher percentage figures. But per cent of what, and excluding what - all PFI one assumes. The CIA quoted on the unreliable Wikipedia has a huge total putting Britain only just behind the USA while monthly figures available today were horrendous. The Devil's Kitchen blog, here, has some other figures from the EU but they make little sense either.... so what is the true magnitude.... surely there is an authoriative internet source somewhere?????