From the story:
Smith told AP he protected the identity of the source he had obtained the documents from by typing copies of them on plain paper and destroying the originals.
I first photocopied them to ensure they were on our paper and returned the originals, which were on government paper and therefore government property, to the source, he added.
http://rawstory.com/news/2005/Backstory_Confirming_the_Downing_Street_0614.html