Identity impersonation with Active Directory Forms authentication

I've got a simple web application (.net 4) using Active Directory Forms authentication & authentication is all OK. The authentication is using the AD authentication provider

I've got impersonation code running in the global.asax and can see using Response.Write(System.Security.Principal.WindowsIdentity.GetCurrent().Name); that the identity appears to be the correct AD identity.

If I try to load a secured root folder (local or UNC) within the impersonation code, FileUltimate throws a cannot access folder error

Any guidance / code example on impersonation to allow access to the secured folders?
(Note - windows authentication in the web.config is not an option - the application has to be forms authentication)

Put the application pool into 'Classic' Mode instead of integrated pipeline