Hi John1
I have cut back to the original cables in the back box
the prev owner had used crimping "bits" then extended the cable
In essence there shouldn't be anything wrong with what you've done. As everyone else has already pointed out, your first point of call should be to remove the socket front and inspect the wiring that's behind it. If you haven't been very careful when pushing the individual wires back into place, one could have become caught behind the accessory screws. This could result in the screw breaking through the plastic insulation to the inner copper core of the wire, causing a short circuit. If this has happened then it should be pretty easy to spot.