We're with Staywarm, I check the tariff every year when it comes up for renewal and for us it's been the best deal plus having the advantage of the charge being fixed for a year so that our monthly outgoings are the same over the course of the year, no sudden price increases without warning.
My husband is in his eighties and I think that it's only available to the over 65s. It was originally based on the number of people and number of bedrooms in a household but I think usage may now be part of the calculation, there was a bit of a scandal when they, having been government-backed, decided to withdraw the service from high users and I was quite worried that we would lose the service but they backed down in the face of bad publicity and we're still with them. When gas prices dropped a couple of years ago this was reflected in our charges for the following year which actually dropped but obviously we're expecting a big hike in cost when the new year's charges for our property drop through our door shortly.