Why on my test site i can't chose another radio button?

HTML & CSS
1 
http://www.handbagsonsaleuk.com/checkout/onepage/

the fourth step (Shipping Method) on the page. there are two radio type input, one is Free shipping radio button another is Flat Rate radio button

why i can’t chose the Flat Rate button. how to correct it? thank you.

2

That’s something that a CSS expect should be able to help you with.
The problem seems to come from the Checkout part of the CSS of your styles.css file.