How to address an Honourable

Today I had an email from an Honourable (as in 'the Honourable Joe Bloggs') and momentarily hesitated over the salutation in the reply. Googling 'forms of address' took me to Debrett's Online, which is full of fascinating stuff from the social season to everyday etiquette. It took ages for the relevant bit to appear on the screen (not sure if the site is very slow, or if it was my computer having an after-lunch nap), but I can now reveal that:
"All sons of a viscount have the courtesy title of 'The Honourable' before their forename and surname. The style of 'The Honourable' (usually abbreviated to 'The Hon') is only used on the envelope in correspondence, in written descriptions (usually only on the first mention) or in formal documents.  It is never used in conversation or on invitations or on visiting cards, when the correct style is 'Mr'.

How to address the Sons of a Viscount

The recommended (social) style of address is as follows:

Beginning of letter	Dear Mr Brown
End of letter	Yours sincerely
Envelope	The Hon John Brown
Verbal communication	Mr Brown (or appropriate rank if in the Services)
Invitation	Mr John Brown
Description in conversation	Mr Brown
List of Directors or Patrons Hon John Brown	Hon John Brown
Place card	Mr John Brown
Legal document	John William   Brown Esquire commonly called The Hon John Brown"

I bet you all wanted to know that.