Set Page Scroll to show content in Fixed div

Set Page Scroll to show content in Fixed div

HTML5 is awesome! Bit to learn with formatting for a novice like me however.
I have a layout page in MVC4 and it is using two fixed div's. One is a
menu and so on and the other is a content holder.
My css snipit:
.maincontent
{
position: fixed;
top:7px;
left:382px;
right:7px;
width: auto;
height: auto;
}
I want to auto scroll the page if this div overflows. I have tried:
overflow-y:auto;
in both the body css and maincontent css elements but its not working.
Am I missing something?
Thanks