Great - thanks. Got it working
For the benefit of others that need to achieve the same:
1. I created a snippet called clientredirect.
2. In it I put:
<?php
$profile = $modx->user->getOne(’Profile’);
$page = $profile->get(’fax’);
$modx->sendRedirect($modx->makeUrl($page));
?>
3. I added the resource Id of the client page in the fax box of their profile
4. I created a resource called clientredirect which runs the snippet - [[!ClientRedirect]]
Job done. Not as elegant as it could be (I tried getting in the extended field to work but was still getting the get() not working).
Thanks so much for your help.
Richard