Following Mary's advice would give you the thread: http://moodle.org/mod/forum/discuss.php?d=105926 read carefully all the snippets from Howard there before proceeding.
But first, moodlers (yes, admins) have been known to delete the admin block by accident!
So can you turn on editing on the front page?
Can you then ADD a block: 'Site Administration'?
In which case, you do not have the same problem as Myles - who may well need to resort to the 'big hammer' from Howard Miller!
Thank you John! Wow was that easy. I never remember deleting the admin block but maybe my helper admin did by accident. Thanks for the fix! Question, I already tried Howard's hammer last night before reading your post. Does that mean ALL my custom roles will have defaulted back? Or just Admin? (which is fine). THanks again and again - saved me!