You can NOT use that old bit of 2 core. You need to replace it with proper twin and earth cable.
Anything else is just a dangerous lash up.
Why would it be dangerous ?
You're increasing the capacity of the live supply without increasing the capacity of the safety earth.
You wouldn't slap a dirty great big engine into a car without upgrading the brakes so you can still stop would you?
Point taken RF, thanks for the replies everyone.