I am replying to my own question as MS support has answered the question
It seems that I had done everything quite correctly.  However, Outlook gave me no clue that I had!  With the email address in the contact record set to InternetFormat= PlainText then messages sent to email accounts over the internet (rather than to a mail server like MS Exchange) get converted to Plain Text during delivery.  On screen the message appears in my default message format (HTML).  It remains in HTML in the 'Sent' folder.  Actually it is quite clever, because if the message has several recipients then the format is only converted for those specified.  It is just that this is quite invisible to the sender.  I had to set up a test contact with my own address to see it work
Outlook Help (on my PC and on the web) was quite unhelpful
   
    ----- ChrisBabe wrote: ----
   
    Outlook XP.  Using Word XP ast default email editor
    Just 1 of my contacts is constrained to receiving mail in Plain Text (using Outlook Express).  I am attempting to default his address book entry (in the integral Outlook address book).  I have found how to set a default setting to 'Plain Text' for this contact [which is to double click on the email address in the contact record; then select 'send plain text only' in the Internet Format drop down box]
    However, when I create a mesage for this contact the format always comes up as HTML.  Even if I create the new message directly from the contact's address book entry
    I can of course set the format when composing the message - but I may forget sometimes - and this defeats the point of having a contact default (different from my generic default which is HTML.  There is no problem when I reply to his messages as the reply uses the format of the incoming mesage
    Does anyone have a solution for this