I have seen electricians argue over this. Some feel the two 3.5mm brass screws are ample and there is no need for extra earth wire others point out that until the item is removed one does not know if socket faulty or no power so back box should be earthed with small bit of cable. It does get heated at times but unless to earth the back box means there are far too many cables going into the earth connection then I see no reason not to connect.
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