You might consider two summer weight sleeping bags rather than one heavier one. That's the method I've opted to go with, and it works quite well. I have two bags which pack down to practically nothing and are rated to about +7C each. (In reality, I wouldn't recommend using just one of them in temps below +10C ... those ratings are stretching it a bit) But with two bags, I've got the option of using one as an additional mattress and the other as a blanket on warmer nights, or one inside the other on colder nights.

I also bring a foil emergency bivy to line the inside of the tent, which adds a little bit to the warmth of the tent, a tiny silk bag liner in case it is cold enough to need it, and a sarong which I use as a sheet at night (and a skirt during the day, or towel sometimes, etc.).