Get an Episerver Page External URL for redirect back from another site

I need to get the external url of an Episerver page for sending to another website for use in a redirect back to my site. 

Is there built-in functionality to do this, or do I need to create a custom function such as GetExternalUrl(PageReference page)?

The external URL should look like "mysiteurl.com/somepage" instead of the relative "/somepage" URL.

Further Information: my site has the Episerver.Web.Current.SiteDefinition.SiteURL = http://mysiteurl.com, but can be accessed locally using mysiteurl.local via the additional hostname. There is also a * wildcard for scheduled jobs. Will this allow the GetExternalUrl(PageReference page) method to return the relative external link depending on the current site, e.g. if on mysiteurl.local and call GetExternalUrl(page) the link will use the .local external link instead of the .com

Any help would be greatly appreciated.

Thanks,

Peter