Interesting article on carb heat research in this quarter's GASCO safety bulletin. Private company at Oxford and CAA conducting research into types of carb ice (butterfly and venturi) and how to prevent them. There is evidence that Carb Ice can cause "rich cut" on the engine - essentially the ice builds up in the venturi and cools everything down as well as causing venturi effect to speed up. So - too much fuel in the mixture because there's less air to carry it and the engine cuts.
They are also trying to see if they can certify a butterfly heater.......so sometime in the next 50 years that might get past the bollox of red tape we have in Euro certification at present.
And the other thing they say in the article (sorry - no link) is that the warmer months of the year are more likely for carb ice in the UK......but it still happens in the colder months. It needs humidity - so the very cold air of the past month would probably not have produce carb ice. But they warn about flying too close to clouds where carb icing conditions are more likely.