From the City of London Corporation website: Details of confirmed and provisional Orders are on the Local Land Charges Register and in the Department of the Built Environment at the Guildhall. A Land Charges search before you purchase a property should disclose the existence of a TPO (or whether your property is in a conservation area). Ask your solicitor if any trees in the property you intend to purchase has a TPO. For advice follow the Contact the City Corporation on the website or call 020 7332 1710.
Go to this page and follow the links for information on Conservation areas