Finally got around to posting the solution i found, in case someone else runs into this problem :)
I managed to find a link on MSDN that put me on the right track and constructed the following console program that sucessfully decrypted the response I had from our fiddler sessions.
I installed the WSE 2.0 NuGet package
try
{
// http://msdn.microsoft.com/en-us/library/aa529137.aspx
XmlDocument response = new XmlDocument();
response.Load("Response.xml");
var encryptedKeyElement = response.GetElementsByTagName("xenc:EncryptedKey")[0] as XmlElement;
var encryptedDataElement = response.GetElementsByTagName("xenc:EncryptedData")[0] as XmlElement;
EncryptedKey encryptedKey = new EncryptedKey(encryptedKeyElement);
EncryptedData data = new EncryptedData(encryptedDataElement, encryptedKey);
var decryptedData = data.Decrypt();
}
catch (Exception ex)
{
Console.WriteLine(ex.ToString());
Console.WriteLine("Press any key to exit");
Console.Read();
}