Use the carb heat! Air temperature isn't the whole story...
Talk to your instructor next time you fly about this, but in the meantime, read CAA Safety Sense Leaflet 14 on engine icing. It should explain all. It's in LASORS - you can either borrow or buy a hardcopy or else download it from the CAA website:
http://www.caa.co.uk/docs/33/LASORS_06_WEB.pdf