Negative battery terminal

In some sort of protest for putting it up for sale the battery has died on me despite sticking it on a conditioner whilst in its carcoon. what fun I've had jacking up the car to get to it but it looks like there is some rather strange looking connector to the negative side I've tried twisting it but it does not seem to want to move. So I've stopped for a cup of tea and thought I would ask the collected experts what I do, can anyone help?

Problem solved the wire itself was preventing a full turn! Doh! Chose the wrong week to give up glue sniffing