Hi all this was almost what I needed but because I was working with a custom theme the "Featured Image" was not attached to a post but to a custom built "Link Widget". Basically I lacked the ability to create a custom field for the url. So I found another solution which others might adapt, learn from or indeed find easier!
What I did was put the url I wanted in the description area when you add a Featured Image. Just pasted it straight in there! Then I ...
Setup the arguments to use with the current post using the current post id (i used a custom variable $number) and put in an array
$at_args = array( 'post_type' => 'attachment', 'post_parent' => $number );
/* please not Ive used $number but you may want to use $post->ID as this was a custom function I was using */
Use the array of arguments use the get_posts() function to get attachment ID data
$attachment_data = get_posts( $at_args );
You cant use the ID by default with get_posts() so we must call setup_postdata() and loop through each array item
foreach( $attachment_data as $post ) : setup_postdata($post);
Stick the attachment ID in a variable
$attachment_id = $attachment_data->ID;
Use the get_post function to call in an array of post data with the corresponding ID (attachment)
$link = get_post( $attachment_id );
Then finally to display the damn url
<a href="<?php echo $link->post_content; ?>">whatever</a>
Hmmm once I right it down it doesnt look easier but may help someone!