how to slim down a hedge which is too wide

According to my book, privet can be cut down to a foot high and you can start all over again with it, this time keeping it narrower as it grows up. You'd need to feed it after you've cut it down. Otherwise you'd have to cut into the old wood to the sides to reduce its width - it'll look awful, be difficult to do, but it should regrow. If it were me, I'd cut it right down to a foot and shape it as it regrows. But you'd better check you can - lots of boroughs in UK have rules about hedges, so a word with the council might be in order if you're going to do something so drastic.