You could try for more.
I am just coming to the end of three months were they moved me up from 4mb to 10mb and dropped the price to £17.99. Ive heard of others getting 9 months.
I'm going to tell them, I'm not going to move down in speed and pay more, when I can get a good deal from talk talk or whoever, leave me on 10mb for £17.99 or cut me off.
The thing is they have already discounted so many people, when confronted with it they have a hard time saying no, especially when other deals look better (at least on paper).