My ganache has gone really firm in the bowl on the kitchen bench overnight (I guess I should have adjusted the ratio to account for this cold Sydney weather.) Anyway, I know I can melt it in the microwave and it should be good under the fondant but I'm concerned that it will set too hard as a filling; is there some way I can now add cream or something to soften it?

You can whip it with your mixer.  Once it is whipped, it will have a lighter "feel" because you have incorporated air bubbles. 

Whipped ganache is sooooooo good it's like eating chocolate air! It will also double so yummy leftovers! Crumb up your cake scraps throw a ton on top stir and chill. I love to throw in chocolate chips to give it a bit of crunch, oh my gosh, pure heaven!

@ElizabethsCakeCreations - That would make a wonderful filling.

 I make a raspberry whipped cream frosting that is to die for. I make my scraps with it, the ganache and the chocolate chips. I think it's the best thing I've ever eaten!  

Hmmm.... Seems like I must give whipped ganache a try as a filling. I'll experiment with my smallest cake first. Meanwhile I've melted half the ganache and added a little more cream that I had heated to the same temperature as the ganache. (I was very careful to measure this accurately - with two fingers.

Hmmm.... Seems like I must give whipped ganache a try as a filling. I'll experiment with my smallest cake first. Meanwhile I've melted half the ganache and added a little more cream that I had heated to the same temperature as the ganache. (I was very careful to measure this accurately - with two fingers.

Hmmm.... Seems like I must give whipped ganache a try as a filling. I'll experiment with my smallest cake first. Meanwhile I've melted half the ganache and added a little more cream that I had heated to the same temperature as the ganache. (I was very careful to measure this accurately - with two fingers.