Just read the paper. (Not just the letter -- the PDF I linked above). None of my research is really all that interesting. Just confirmation from a variety of sources all reporting the same thing. The most different is by the Sunday Herald (of Glasgow).

The most interesting thing not reported in the Spiegel article and not in the paper itself is that a UK MP, Tam Dalyell (Labour party leader in the House of Commons, if I understand the terminology properly), apparently called out Blair for going along with these people.