Table of Contents not matching chapter

Hi,

I'm editing a book containing four chapters (individual documents) and an introduction which includes a table of contents.

The ToC displays the contents by looking at the heading styles of the four chapters as required no problems - 1234.

but looking at the numbers of the chapter headings i get 1567.

When i go to the chapters, chapter 1 is fine but chapter two heading style shows as "5" not two and the subsequent chapters follow on from that. 

if i remove the ToC, chapter two heading number reverts to 1.

Its like the heading in chapter two is getting its number from the end of the ToC and not chapter one as it should.

I've tried various fixes, used a different style for the ToC entries, re-applied styles, checked section breaks etc.

Can't for the life of me get to he root of the problem.

Help appreciated.

thanks.