At 15C you should almost be able to start the bike without the choke if it still has carbs and the carbs are working OK.
Check the health of the battery. A fully charged battery should have around 13.2V when doing nothing, so 12V is a little low. Did you by any chance measure the voltage when cranking the engine? That's usually a good indicator as to how healthy the battery is. Anything below 10V would indicate that it is time to replace the battery, especially if it has been considerably discharge a few times. My main bike was suffering from that last year, it was cranking OK but wouldn't fire because the battery was close to giving up the ghost. A new battery solved that problem.
I would also check or get someone to check that you have a nice fat spark at the plugs. If the engine has been flooded, I would change the plugs - when they get flooded, a lot of plugs don't seem to recover too well. I've had this on several vehicles and usually spending a couple of bucks for new plugs resolved the issue.
Also, try to start the engine with the fuel filler cap off. A blocked vent can lead to the fuel not flowing properly to the carb as the fuel that is trying to exit the tank is creating a vacuum in the tank.