SignedXml - SHA256

Hi

I've got SignedXml working with SHA1.  I've now got a SHA256 certifiate and I want to use SigedXml to generate the Digital Signature using SHA256.  I've tried quite a few ways of doing it.  However without success. 

I've added            

CryptoConfig.AddAlgorithm(typeof(System.Deployment.Internal.CodeSigning.RSAPKCS1SHA256SignatureDescription),
                "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256");

to the proejct but I'm still getting errors and this one seems to be the common theme.

{"Could not create hash algorithm object."} 

Any ideas if I can do it?

Thanks in advance