Complex sentence: The contents of the memo were leaked to the UK press in 2005-its authenticity has never been seriously challenged although the US and UK governments have both attempted to argue that the memo does not accurately reflect what their official policy positions were at the time.
Split into simpler sentences: The contents of the memo were leaked to the UK press in 2005. Whilst its authenticity has never been seriously challenged, the British and American governments have both attempted to argue that the contents do not accurately reflect their official policy positions at the time.