Alternatively, you can address your letter to a particular department of the company, e.g. The Sales Department, The Accounts Department.
In both these cases an organisation name should be included as part of the address, as the address to which you are writing could be home to a several companies, situated in the same building and using a communal post room; therefore there could be several, say, personnel managers. So the correct form would be:
Mr John Smith
Personnel Manager
Avco Tools plc.
COMPANY KNOWN
Finally, if you know nothing about the company and do not know which person or department your letter should go to, you can simply address the letter to the company itself, e.g. Compuvision Ltd, Messrs Collier, Clark & Co.
The organisation’s name should be given as the version used in your correspondent’s letterheading, including any designation of status, such as Ltd or plc. If writing to a partnership the form ‘Messrs Price & Green’ is correct.