I top-post my belated reply to conform with custom in this newsgroup.
Putting the last message after the sig is new to me and I hope it works.

It took me a long time to make sense of your valuable reply. I did not
see "Idiots' Guide to Installing Macros" at the bottom of the view
rendered on opening <http://www.gmayor.com/installing_macro.htm>

AutoOpen runs automatically on opening my main document and when it
opens the output document created when I click Edit individual
letters... (I infer this - I have yet to work out how to insert a
breakpoint in the code of a VBA module. I used to do this in C but am
unable to remember the technique. I have some memory of 3 as a function
parameter.) I use Edit... because Print... does not successfully produce
double-sided output on my printer.

I suppose I need a condition on ActiveDocument.Fields.Update which would
suppress it on the latter document. I have yet to find that condition
and hope for further assistance.

I got a copy of "Office VBA: Macros You Can Use Today" and applied Cindy
Meister's "Mail Merge: Using a Relative Path for Data Source" procedure.
(I now use the complicated version which opens Sheet1 automatically.)
I added the line
ActiveDocument.MailMerge.ShowWizard 5 'Open "Preview your Letters" pane
I have yet to find out how to automate "Edit recipient list..."

When I use that control, the order of columns in the window which opens
is not the same as in my data source. (The first data column is
"Postcode"; it is column J of my Excel data source.) I see the same
behavior (sic) with both the default and Cindy's version of "Use an
existing list". I hope this is an FAQ and would value a pointer to it.

I am sorry this is a portmanteau response. I am trying to run when
barely able to toddle. ;)