How are you sending the message? Email or Snail mail?
How did you set up your form letter capability exactly? (There's simple and more complex options for how you might do this.)
I am sending the message by snail mail. If I understand your question correctly I set up the letter as a new layout? Hope that makes sense.
The simplest approach to producing form letters is to combine layout text with merge fields on a layout and then you can print a bunch of form letters that all have the same layout text, but have data from your database inserted into the layout text to produce a letter. To a limited degree, you could make such a letter "customizable" if you added a merge field at the beginning or end of the letter that was blank for records for which you don't want to add that "customized message". You could then edit that field in a different layout to provide "custom text" for just one recipient.
But that's not the only option. You can also set up your basic message as Text in the record of a "template table" where each record is a different standard message with place holder text inserted for different fields. You then use a layout with a large calculation field that uses the Substitute function to replace the placeholders with data from your database to print out your letters. Since the message body is now text in a field, you can duplicate the record and edit that text to produce a different form letter from that created originally.
See this demo file for a simple example of such a message template system: https://www.dropbox.com/s/6xw8buafjuohncu/MessageTemplateBuilder.fp7
FileMaker 12 and 13 users can use Open from the File menu to get a copy converted to the .fmp12 file format.
Thank you Phil(?) this is most helpful. I am away from my desk today but will take on board your suggestions later when I have more time to digest them. Kind regards, Andrew, Hay on Wye, UK