This rank is held for life. A Field Marshal would almost certainly be a peer, baronet or knight.
It should be ascertained whether he prefers his military rank to be used or his title.
How to Address a Field Marshal
The recommended social style of address is as follows (if a peer):
|Beginning of letter||Dear Lord Blank|
|End of letter||Yours sincerely|
|Envelope||Field Marshal Lord Blank, GCB|
|Verbal communication||Lord Blank*|
|Invitation||Field Marshal Lord Blank|
|Joint invitation+||Field Marshal Lord Blank and Lady Blank|
|Description in conversation||Lord Blank (or The Field Marshal)|
|List of Directors or Patrons||Field Marshal Lord Blank, GCB|
|Place card||Field Marshal Lord Blank|
*A younger man, or a more junior officer in any of the Armed Forces, addresses him as 'Sir'.
+ Note: Traditionally, invitations to a married couple, when sent to their home address, are addressed to the wife alone, with both names being inscribed on the invitation card. It has become increasingly acceptable, however, to address the envelope with both names.