Here are some APPROXIMATE comparisons between the difficulty levels of SuDokus on this website and those of some popular UK newspapers.
The Times 'Easy' & 'Mild'
The Times 'Difficult' & 'Fiendish'
The Independent 'Advanced'
The Daily Telegraph 'Moderate'
The Times 'Super Fiendish' (most)
The Daily Telegraph 'Tough'
The Times 'Super Fiendish' (hardest)
The Daily Telegraph 'Diabolical'
Beyond most SuDokus found in newspapers or puzzle books