I think they originally started off on the electricity adverts many moons ago, athough I could be completely wrong! Just lately, a new breed has started advertising tea bags - Typhoo or PG tips?
Yes, they were for electricity. HeatElectric and CookElectric. The characters weren't invented for the ads - the short film came first, in about 1989. Both the original short and the full set of adverts are available on a video called "Aardman Animations Vol.1 Special Edition".
They made it to T-shirts you know. Someone I know still has theirs, and on the front is the tortoise saying 'easily turn off and onable'! My Dad has one too - I'm sure he's never worn it - and the phrase on that one was 'I'm a very busy person', but I can't remember the creature that went with it! Brilliant stuff!
Here's a bit of marketing history for you. The original creature comfoty ads were for Electricity and were one of the most successful campaigns of their time however, when asked what they represented the majority of the population thought they were for gas so their success was in fact for the wrong item. Bet they couldn't do that again if they tried.