Centering Nav menu in header

Hi, I am trying to centre a nav menu in my header but cant seem to stop it from sitting on the left, can anyone see why? here is what I hope is the relevent bit of code:

#nav            { margin-right: auto; margin-left: auto; width: 990px; height: 60px; padding-top: 30px; padding-bottom: 2px; border-bottom: 1px solid #fff }
#nav li      { list-style: none; margin-bottom: 30px;float: left }
#nav li a          { color: #fff; font-size: 150%; font-weight: 450; text-transform: uppercase; z-index: 999 }
#nav li a:hover     { color: #fff; text-decoration: none; z-index: 999; -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=80)"; filter: alpha(opacity=80) }

#nav .parent > a, #nav .parent > a:hover{ background: #fff; background-position:right;background-repeat:no-repeat; z-index: 999;}
#nav ul, #nav ul li{display:inline;list-style:none;padding:0;margin:0}
#nav ul li a{display:inline-block; text-decoration:none; padding: 0px 0px 0px 0px; z-index: 999;}
#nav ul li a:hover {  }
#nav ul #first a:hover{border-radius:5px 0 0 5px; z-index: 999;}
#nav ul li{position:relative}
#nav li ul { display: none; position: absolute; top: 51px; left: 11px; z-index: 999; }
#nav li ul a   {  z-index: 999; -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=80)"; filter: alpha(opacity=80); opacity: .8 }
#nav ul ul li:hover ul, #nav ul ul ul li:hover ul { left: 0 }
#nav ul li:hover ul{display:inline-block}
#nav ul ul, #nav ul li:hover ul ul, #nav ul ul li:hover ul ul {}
#nav ul ul ul li { margin-top: -14px; }
#nav ul li:hover ul, #nav ul ul li:hover ul, #nav ul ul ul li:hover ul { display:block; z-index: 999;}
#nav ul li:hover ul li a, #nav ul ul li:hover ul li a, #nav ul ul ul li:hover ul li a{ margin:-1px 0 0 -11px}
#nav ul li:hover ul li a { padding: 5px 14px 15px; width: 150px }
#nav ul ul li:hover ul li a {  }
#nav ul ul ul li:hover ul li a{width:140px;padding:0 14px 0 24px}
#nav .children li a:hover     { color: #fff; background-image: none }

I should say I am also adding the custom background images to each label by injecting them in the menus/custom links/label box as you will see on my site

https://www.fnordcorps.com