Non-Driver. I've got the same problem with TalkTalk. Their latest reply to my complaint was:
I have looked into your account and can see that you have been on Tiscali Max 40GB plus Anytime Talk II since December of 2009. In your July invoice your subscription was 19.99 and in your August invoice a price increase is showing from 20th of July. This brings your monthly subscription to £22.48. If you are not happy with this change in your price plan, I have to advise you to contact our Re-pricing team. They may be able to put you back on your old price plan.
One of our guiding principals as Tiscali becomes part of the TalkTalk group is to ensure we are transparent. With that principal in mind, we have aligned all of our product packages and pricing within the TalkTalk Group. This means some customers will get a price decrease, some customers’ prices will stay the same, and some will receive an increase.
Anyway, I've just telephoned 0800 1105850 and they put me through to the Tiscali desk; result, refund of the additional charge for August and I will get the next 6 Months at the £11.99 introductory rate. I haven't had my confirmation e-mail yet though. Being firm, insistent and indignant but polite seemed to do the trick.