security policy exception

Folks,
I have a winform which amongst other things includes some SQLClient functionality. I have used the .Net framework security configuration tool to create a code group which has SQL Client permissions. I have used the option to apply this code group to all apps run via the local intranet.
The winform runs fine from the server or when installed on a client. When I try to run it from the client via a loader form with assembly.loadfrom(winformurl) or via a shortcut, the form loads but if I try to do anything that uses SQLClient functionality e.g. select or add a record i get a "The application attempted to perform an operation not allowed by the security policy" error.
Can anyone help?
Thanks in advance