I moved house a few weeks ago and got stuck for 6 weeks on a really rubbish tariff whilst i was switched over to my old supplier. After phoning them, all I had to say was 'you did not explain the implications of the tariff or what options I had when I moved in' and they instantly crumbled and refunded 10% of my combined gas and electric bill. Given my gas bill for 1 month was just under £500 that made a little difference!
I think there is a genuine fear about regulatory headlines leading to another rush of mass-claims. Dealing swiftly and proactively with issues on demand will likely be an easier route and demonstrates good faith to the regulators.
It also reminds me of when there was that fuel-contamination saga a few years ago when engines got ruined by dodgy petrol. My dad had filled up around the time the bad fuel was in circulation but was pretty sure it was a good couple of weeks after the fuel had been purged. He phoned Tescos to say he didn't think it was related but his car head gasket had blown shortly after filling up and could they just confirm the dates that the fuel was defective. Tescos insisted he speak to the solicitors dealing with claims. My dad said he didn't want to claim, but just wanted to understand the dates - he was pretty sure it was ages afterwards. Tesco insisted he speak to the lawyers. My dad emailed them and a few days later received a letter saying his claim had been unsuccessful. He then emailed back to say he just wanted to know the dates that the fuel had been defective. A week or so later he received a letter saying his appeal had been successful and enclosing a cheque for £1,500.