THE merger mania in the British electricity industry during the summer cost generating giant National Power almost £60 million sterling, it emerged yesterday. The company said the cost of its abortive acquisition of regional electricity company Southern Electric and its defence against a takeover bid by Southern Company of America had cost it £57 million. The outlay dimmed pre tax profits to £194 million over the six months to September 30th compared with £254 million in the corresponding 1995 period. Shareholders will be protected from the fall and will see their interim dividend rise 11 per cent to 6p from 5.4p.