Hi Ant,
Thanks for your response. I was taking a closer look today. You can search by email if you belong to the moderator member group. I believe this works in 2010 but you must belong to a moderator member group. It would not work previously if you only belonged to the admin member group for example.
In 2012 we've resolved this to ensure default behaviour for both moderators and admins is to search both username and email address within the Admin CP.