Image sometimes not found - path changes

Hi All. After searching the many entries for similar problems, I can't find an entry that is the same as my problem. First the background:

Drupal 6.19, Zen subtheme using fresh.css, LAMP Ubuntu 9.10, clean URLs enabled

site: https://smhs-leeds.co.uk

I have created a div in the header for a 2nd logo on the right. Most of the time the image loads correctly. Sometimes it doesnt.

In my page.tpl.php I've got:

<div id="right-logo">
            <a href="http://www.nhs.uk/" title="NHS Choices"><img src="sites/all/themes/intra2/images/nhs-logo.png" alt="NHS Choices" /></a>
          </div>

Interestingly, when I visit /calendar and /documents, the image loads (views paths). When I visit a node the image breaks due to it trying to load the image from node/sites/all etc. When I view a forum, similarly, it tries to load image from forum/sites/all etc. There are other places it breaks too.

I'm thinking its maybe a URL rewrite problem, but have no idea where to look next. Any clues?

Thanks in advance

Mark