Yep, rock_shoes is right. It does sound like an air leak. It could also be a bad lift pump.
Go to the sticky on air leaks at the top of the 6.2 forum. In that thread there are directions on how to find an air leak. Do the test to see if there is a leak. If you find one, fix it so it doesn't leak anymore and see what happens...
If there aren't any leaks, you may have to replace the lift pump. Sometimes if the lift pump is bad it will allow fuel to drain back to the fuel tank and then it will take a lot of cranking to get it started again, but if you can smell diesel fuel it sounds like a leak so it probably isn't the lift pump.
You are right... 10 degrees C is warm enough you shouldn't need the block heater. I figured it might be cold where you are... I guess I just assumed that because it is cold here... ):h