Word headings - mysterious renumbering of headings

OK I'm trying to do the right thing with Word 2003 headings but keep seeing this...

7. Big heading
1.1 Next heading

When what I want is

7. Big Heading
7.1 Next heading

This has confused most people I know apart from the very clever chap I sit next to, Tim.

If you are like me you will know that you need to do the following to modify the style of 'heading 2'...
Format - Styles and Formatting
Right click on 'heading 2' and choose 'modify'
From modify dialog, click the 'format v' button and choose 'numbering'
From the 'bullets and numbering' screen hit 'customise'
And lo and behold you are at the 'Customise Outline Numbered List' dialog.

From there I could not figure it out and no matter what I tried it did not work.

It turns out that my level 1 headings and level 2 headings were 2 unrelated lists. Tim figured this out by changing from 'Level 2' in the topmost and leftmost position on the dialog, to 'level 1'. The 'link level to style' field was NOT set to heading 1.

Sounds simple but unless you realise that you need to change the level in the top left to level 1, you will miss this like me and about 5 other people did (all experienced Word users).