Length of API key string?

Hi

This might be a very simple question to answer. I have a 40 char API key as provided through this portal, however it does not appear to work (repeated 400 error codes). Chatting with a friend, he explained he had a 56 char API key that he uses successfully, and I copied it into my code and worked first time. Am I missing something from my API key to make it work?

I am registered for the REST API key.

Sorry if this is a stupid question.
Phil.

I have messaged you privately.