Thousands of drivers who have failed to pay the new Greater London Ulez have been sent warning letters rather than £180 fines, it can be revealed.Transport for London has confirmed that many motorists have escaped an immediate penalty as it pursues a phased approach to enforcing the Londonwide ultra-low emission zone.Letters headed “Ultra Low Emission Zone contravention warning notice” have begun arriving at the homes or workplaces of drivers who failed to pay the £12.50 daily levy this week.