My son is doing the same thing. He is older (nearly 17 months now) and I am thinking of putting a cot bumper around the sides of his cot.

Have you thought about using a sleeping bag (grobag, living textiles etc) instead of blankets, so he can't throw them off?

There are a few girls in your general region on here who may be more familiar with the weather there than me, who may be able to advise a bit better.