Heading numbering all wrong.

[davemar]

I've written a document, and somehow my heading numbering has gone all wrong. I'm using 3 levels of headers (Header 1, Header 2 and Header 3) with the numbering like this style:
1
1.1
1.2
1.2.1
2
2.1
2.2
3
3.1
3.1.1
etc....

However the numbering is all over the place. Here's my table of contents (names & page numbers removed):

1
2
3
3.1
1.2
1.3
1
2
3
3.2
3.2.1
1.4
1.5
1.6
1.7
3.2.2
3.2.3
3.2.4
4
1.8
1.9
5
1.10
1.11
1.12
6
1.13
1.14
1.15
1.16
7
8

So it is completely bonkers!

What has happened, and how do I get it to do something sensible?