How to use SharedAccessSignature to access blocks

I am trying to access a blob stored in a closed container in Windows Azure. The container has a shared signature, but when I try to access the blob I get a StorgeClientException message "The server could not authenticate the request. Make sure the authorization header is configured correctly by including the signature."

The code that created the container and loaded blob is as follows:

// create the container, set a Shared Access Signature, and share it 

// first this to do is to create the connnection to the storage account
// this should be in app.config but as this isa test it will just be implemented
// here: 
// add a reference to Microsoft.WindowsAzure.StorageClient 
// and Microsoft.WindowsAzure.StorageClient set up the objects
//storageAccount = CloudStorageAccount.DevelopmentStorageAccount;

storageAccount = CloudStorageAccount.Parse(ConfigurationManager.AppSettings["ConnectionString"]);
blobClient = storageAccount.CreateCloudBlobClient();

// get a reference tot he container for the shared access signature
container = blobClient.GetContainerReference("blobcontainer");
container.CreateIfNotExist();

// now create the permissions policy to use and a public access setting
var permissions = container.GetPermissions();
permissions.SharedAccessPolicies.Remove("accesspolicy");
permissions.SharedAccessPolicies.Add("accesspolicy", new SharedAccessPolicy
                                                               {
                                                                   // this policy is live immediately
                                                                   // if the policy should be delatyed then use:
                                                                   //SharedAccessStartTime = DateTime.Now.Add(T); where T is some timespan
                                                                   SharedAccessExpiryTime =
                                                                       DateTime.UtcNow.AddYears(2),
                                                                   Permissions =
                                                                       SharedAccessPermissions.Read | SharedAccessPermissions.Write
                                                               });

// turn off public access
permissions.PublicAccess = BlobContainerPublicAccessType.Off;

// set the permission on the ocntianer
container.SetPermissions(permissions);

 var sas = container.GetSharedAccessSignature(new SharedAccessPolicy(), "accesspolicy");


StorageCredentialsSharedAccessSignature credentials = new StorageCredentialsSharedAccessSignature(sas);
CloudBlobClient client = new CloudBlobClient(storageAccount.BlobEndpoint,
                                             new StorageCredentialsSharedAccessSignature(sas));

CloudBlob sasblob = client.GetBlobReference("blobcontainer/someblob.txt");
sasblob.UploadText("I want to read this text via a rest call");

// write the SAS to file so I can use it later in other apps
using (var writer = new StreamWriter(@"C:\policy.txt"))
{
    writer.WriteLine(container.GetSharedAccessSignature(new SharedAccessPolicy(), "securedblobpolicy"));
}

The code I tried to use to read blob looks like this:

// the storace credentials shared access signature is copied directly from the text file "c:\policy.txt"
CloudBlobClient client = new CloudBlobClient("https://my.azurestorage.windows.net/", new StorageCredentialsSharedAccessSignature("?sr=c&si=accesspolicy&sig=0PMoXpht2TF1Jr0uYPfUQnLaPMiXrqegmjYzeg69%2FCI%3D"));

CloudBlob blob = client.GetBlobReference("blobcontainer/someblob.txt");

Console.WriteLine(blob.DownloadText());
Console.ReadLine();

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