We have a double socket about three feet above floor level, set in a lath and plaster wall. Never been right since we moved in, the only fixings being screws through the back box into the laths behind. Long story short, only one of the screws is really secure, the others being too close to the edge of a lath, which splits.
Today, I used a long bolt to secure a three hole strip of Meccano behind the back box, and 'toggled' it into position such that the Meccano is
45/225 degrees behind the lath/plaster, and, when tightened, together with a screw fixing in the opposite corner, gives a good solid fixing of the back box. Wifey is delighted.