Hi Allistair,
Thanks for posting your question. Here is a suggestion that should resolve the issue.
The first time selecting to email the link from the record in CRM 2011, there should be a prompt for which application to use. However, if you set the Metro Mail app to be the application, it will set this application as the default program.
Here are the steps I applied to switch the Mailto URL protocol from the Metro Mail app to Outlook 2013.
1. Navigate to the control panel, or press Windows key and type default
2. Here is the path within the control panel of the Windows 8 machine to apply the specific program for the file association: Control Panel\Programs\Default Programs\Set Associations
3. Scroll down to the bottom of the list into the Protocol section
4. Find the MAILTO protocol, with the description URL:MailTo Protocol, double click and change to Office 2013
Hope this helps,
Al Ericson