Hi Mummymeg, Charli had this too. The Child health nurse said they see this so commonly these days because with the SIDS recomendations babies don't sleep on their tummies like they used to.
Get the paed to check it out just to be sure.
There is something you can buy to put under bubs head when they sleep, I think it is sort of a wedge shape thingy and gently makes them face their head slightly the opposite way. I think baby shops sell them. But it was pointless us getting one for Charli because she was too old and moved around in the cot too much by the time I found out these things existed.
Like someone else said, it's good to give her lots of tummy time to take the pressure off that part of her head, or perhaps when you have her sitting up in a rocker/bouncer you could gently put a bit of a rolled up hand towell or old cloth nappy under the side she usually favours so her head is facing slightly the other way, and to again, alleviate a bit of pressure off the flat spot for a little while.
I've found with Charli that since she has been able to sit up her head has gotten much better.

HTH