Many thanks for your messages but I'm not sure that this is the case, this works from time to time - Surely it would be an all or nothing scenario for the double hop? The process is called the same way every time.

Both servers have SPN's registered and the sql account on both servers is registered for delegation. The only part that may not be configured is the machines themselves as they are not set to delegate in AD, but I am not sure that is necessary.