+OK, I give up I can't get it to work.
The way I solved it was to divide the document into 9 sections and then manually insert the different page numbers in both the headers and the footers.


That took 18 numbers in total and less than a minute. I know in case of a 100+ page document this is no solution but this task has never come up and I don't think it will too often.
Thanks anyway.

Update:
This problem just didn't leave me alone.
What I have found (I hope I'm not wrong) is that the code
{={PAGE}+{Section 1}} can be manipulated like this:
{={PAGE}+x} or {={PAGE}-x}
where x is any integer number,
which makes it very easy to solve my problem.

Yes, you could do that, but then you have to update 'x' every time the pagination changes due to edits. And, if you have any Sections that end immediately before/after an automatic page break, simply changing printers could render your 'x' values invalid due to Word optimising the layout for the current printer.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

That's true. In my case we are talking about a translation job and in this case I always pay attention to leave enough space for different printer settings.