Andy, ours was fixed until March, but there was no penalty to leave that one. (btw, anyone can actually leave any plan once you're down to within 49 days, without incurring a penalty)
The one we've just signed up for is until Feb 2017, which has a small penalty, but we won't be changing again within the next 12 months anyway.
There was also a `Fair deal` plan advertised.
We actually rang first to see if they could tell us what the cheapest plan was, but the guy said they aren't allowed to do it over the phone, it had to be done online to get the best deals.
We also did the usual comparisons online too to see which suppliers had good deals & actually wanted to see Ovo (as recommended by others) near the top of the list, but it wasn't even in the top 10 for us.

There were actually a couple of providers slightly cheaper than The co-op, but the difference was minimal + they were providers I hadn't even heard of & had penalties of up to £25 per gas AND leccy, to leave early.