how do I connect my Belling 335?

I cannot work out how to undo the connection facility on the back of the cooker to connect to the mains supply.
Cooker is a Belling 335 WH 50CM. I removed the back panel to see if I could work out how to get at the connections but still can't work it out. There is a black plastic box that I assume has the connection nuts inside but if it does come apart I can't work out how