I've got the Angling Intelligence 3 in 1 Sleep System and find it to be warm whatever the weather.
If you feel the cold, try wearing a base layer to bed as well.
And a woolly hat / balaclava will help a lot as the head is where most of your body heat will escape from when you are in a sleeping bag (They had these in Aldi last week. They were £4.99 each if I remember correctly).
You can also add a pair of Heat Holders socks to complete the ensemble.