Been with them since 2006 and every time I have had a problem they would either fix it instantly or reimburse me until the issue was fixed.
for example One time a few years ago I went over my internet allowance by accident, and they charged me an extra £10 or such for only about 10Mb. I called them up and complained as usually they send you a text reminding you that your allowance is about to run out. That time I didn't receive the text so I wasn't aware. They removed that charge as a gesture of goodwill at the time.