Adding a double socket for TV

Hi I’m looking to add an extra socket in behind my TV so I can hide TV wires behind my wall. Where I want to put the new socket is 3m away from an existing double socket. I’ve opened the existing socket and it has 2 cables in it. Could I tap into it and run a length of 2.5mm twin & earth to the new socket? 

Im afraid that while, with a simplistic view, that this is what it seems is done,,,, that there is a lot more to it than that.... so the simple answer is not unless you can determine that the circuit and supply is safe to carry the additional load, that the earthing and bonding is compliant with the regulations and that the circuit has 30mA RCD protection.

Once you have installed the new cable in the appropriate safe zones and connected it, you then need to be able to carry out certain tests to confirm that it is safe to use... the testers to do this cost  £400 and upwards (mine cost £1200)