Yellowing leaves on Laurel Hedge

I have a magnificent 70 year old laurel hedge which is 9ft high, 3ft deep and 40 long.  It has developed a 3ft section of yellow leaves about a quarter of the way in.  It is yellow top to bottom on the south side and top on the north side.  We have done nothing different to the hedge over the years and it is healthy and flowering everywhere else.  We are in the south east of england and do not experience any real strong weather conditions.  Any idea what could have caused the problem and what if anything I can do to reverse it?

Christine