If anyone else has to do this...
First of all, if you are not confident / competent with electronic work, Get a professional to do this.
Not using the right gauge wire or connecting things up incorrectly or insecurely risks either fire, electric shock or both. Additionally, once opened keep in mind that the units water resistance is likely to be compromised.
You will need a t10 security driver, this has a star head with a hole in the middle. Remove the screws from the base of the unit.
Once these screws are removed you will need to prise the base of the unit off. This was where I struggled, the rubber seal between the unit and the base is secured to both elements. I continued to prise in the same location, eventually the base started to lift off.
Once inside, there are 4 connectors on the mains electric side of the unit, 3 are wired to the mains wires, the other bonds earth to the rest of the unit.
Take a picture / note of which wire goes where, on my unit with the mains electric at the bottom it was
Live, earth, earth, neutral. The left earth was the one that needed disconnecting. There are connectors crimped onto the ends of the wires, these are not reusable and you will need a crimping tool and matching connectors to safely replace, wrapping the new wire round the screw may be adequate, but why chance it?
You will need to put the seal and nut fro. The end of the unit onto your new wire, if you have the right gauge wire, this will be an incredibly snug fit.
Fasten you newly crimped wire to the terminals, fasten the wire in place the the metal bracket that you had to remove earlier, to can test the connection with a multimeter.
Put everything back together and done.
I will only be using my charger now plugged in the garage or in good weather as it may no longer be waterproof. If you so wish I supposed you could reseal the unit with some kind of suitable sealant.