HeaderAuthentica­tionProvider doesn't exist error when browsing to custom application

Microsoft.Crm.WebServices.Crm2007.MultipleOrganizationSoapHeaderAuthenticationProvider, Microsoft.Crm.WebServices, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' doesn't exist.Parameter name: Microsoft.Crm.WebServices.Crm2007.MultipleOrganizationSoapHeaderAuthenticationProvider, Microsoft.Crm.WebServices, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35

Very common error, but i thought i would note it down anyway....
To resolve, retrieve the crm.webservices.dll from the crmweb\bin directory and place in the custom applications bin directory.

Enjoy

Jonathan