Sharepoint controls.css problem

Hi,

I'm working on a Sharepoint site but am having an unusual problem with controls.css.

None of the sharepoint controls are getting styles applied to them so the site is practically unuseable as it is. E.g the Site Actions button has no graphics displayed, it is just a piece of text & the dropdown from it when clicked is cluttered and unstructured. This applies to all controls including the Rich Text Fields etc.

I am importing the controlls.css the regular way with:

<SharePoint:CssRegistration name="<% $SPUrl:~sitecollection/Style Library/~language/Core Styles/controls.css %>" runat="server"/>

and when that didn't work, I tried:

<link href="<% $SPUrl:~SiteCollection/Style Library/en-US/Core Styles/controls.css%>" rel="stylesheet" type="text/css" media="screen" />

I have used firebug and I can see that the css is being imported in the head section because I can actually browse through it's styles.

Any suggestions or advice would be greatly appreciated.

Thanks
John