Related to entries in categories in a plugin
Ok worked this one out eventually myself. If anyone else finds this helpful.

$userinterests = craft()-&gt;users-&gt;getUserById($userid)-&gt;getContent()-&gt;getAttributes();
$criteria = craft()-&gt;elements-&gt;getCriteria(ElementType::Entry);
$criteria-&gt;relatedTo = array(
        'targetElement' =&gt; $userinterests['relevantFieldToCheckAgainst'],
        'field' =&gt; 'EntryFieldToCheckIn'
);
$relatedEntries = craft()-&gt;elements-&gt;findElements($criteria);