I am designing a template in Word for someone to use for letters, and have inserted a different section in the second page - specifically so that the margins in the heading are different. My problem is that when I type on the first page, continuing onto the second, the margin changes in the heading, to match the first section, which is not what I want. As it is for someone else, it is not convenient to ask them to insert a section break every time they finish the first page - particularly because they are using tables which are not easy to break up. Any advice would be appreciated!

You do not want to use a section break, because of the problem you are seeing. You want to use a different first-page header set up so that it looks like margins are different.

Heading (a style) or Header (a part of the document at the top of pages)?

See Setting Up Letter Templates and the Letterhead Textboxes and Styles Tutorial

You might also want to ask on the Word for the Mac forum.