Erratum.
Sorry I misread what you said Paul, thought your ticket said you were landing at Don Muang.
It's highly unlikely that you will be landing there, whatever the booking website says. They probably just haven't updated the website. I read somewhere (maybe the Bangkok Post) earlier this year that a few international charters use that airport. Regular flights all land at the new airport AFAIK.
Doesn't affect taxi instructions for getting into town though it will be slightly more expensive.