I was on that tariff a while ago. Saved a few quid for a while then it got expensive. I bailed out a year or so ago, switched to fixed, now on the standard "capped" tariff.
Guess it's not a good time to re-join, must be something weird going on that's probably short-term. I'm guessing that's the result of it being windy over the weekend, so there's a surplus of power. I saw similar one christmas, it was about 1p a unit on Christmas Day one year.