Just thought I'd update this in case anyone reads the thread in future...
I've just installed the Radian in the car and it's a tight squeeze but it does fit! Yay! I'm 5'3" and I have just enough leg room in the front passenger seat...don't think my 5'11" DP would have any chance, but when we travel together in the Astra he drives anyway. I absolutely love it which are strong words for a carseat but it seriously is awesome! (as lots of you already know!)