From
http://www.home.ntl.com/page/broadbandexisting
Existing 750K customers
We´re in the process of automatically upgrading your service to 2Mb. This upgrade is absolutely free and you don´t have to do anything to receive it.
What´s more, the monthly price of this service remains the same. Check this page again for updates on when you will be upgraded.
I'm in almost the same situation, paying 19.99 a month for 750k but it's because I originally bought my CM. I saw the page that said 2Mb for 24.99 and decided to wait for the automatic upgrade but having seen the above page I might take an offline copy and go ahead and upgrade now.
Just had a thought... the price might stay the same but I lose my discount.... Think I'll wait