The rule is that if you have a British passport you can claim 15 points. If you want to claim the full 25 points you need to obtain a band 7 score for each of the four IELTS tests (Reading, writing, listening & speaking).
The tests aren't difficult however you need to study the provided material to obtain the desired band score. Look at this site for more info http://www.ielts.org/
I have just received State sponsorship and like you I was 5 points short, so I have just completed my IELTS test and will receive my results on 13/06/2009.
REMEMBER that there is a few weeks waiting list for these exams so book ASAP to take them so that when you receive your State sponsorship that you are able to submit your DIAC application within the given 60 days.
All the best