I think it might be a grass snake actually - the pattern that looks like an adders back is actually the belly here. If you look at the back of the snake you see the grass snake markings and the flashes of light colour behind the head as well.
If you have another picture or can take one showing the head and the back of it I can be sure.
Either way they are protected, so don’t harm it in any way. I’d leave it to warm up in the compost heap and sort itself out. Kitty should be fine if they are not showing signs of illness right now.