leaving out cream in gratin recipe - do I need to thicken the milk instead?

[gretagrape]

Hi - I'm doing a gratin tonight which calls for 300mls of double cream and only 4tbsp of milk. I don't want to use cream (partly health, partly the weather is rubbish and I don't want to walk to the shop just for a pot of cream!) - should I just use milk but maybe add some cornflour The recipe says to simmer the ingredients on the hob before putting it in the oven, so I could add the cornflour before putting it all in the baking dish?
Thanks
x

I never use cream, always milk. Never thickened it though.

Thanks - will give it a go.