Information required on company notepaper
This is now covered by the Trading Disclosures provisions in the Companies Act 2006 sec82 - sec85 and the regulations made under those sections.
The information required on all letters, emails, faxes, websites, etc. is
Company's full name
Address of registered office
The fact that registered in England or Scotland, etc.
The names of the directors need not be stated but, if any are (other than as signatory) all must be stated.