OK, I've managed to fix the sidebox width like this:

#manufacturerslistContent {
width:91%
}

#manufacturerslistContent ul {
height:645px;
width:85%;
list-style-image: url(../images/bullet1.gif);
list-style-position:outside;
list-style-type:none;
margin-left:35px;
margin-top:0;
overflow:auto;
padding:0;
}

There wasn't anything in the stylesheet.css for #manufacturerslistContent and that seemed to be the problem.

The only issue I have now with cross browsers is the white line in Chrome, Safari and Opera.