OK, I have just wasted a few hours trying to figure out was was wrong only to discover that it's a bug that has been known for over three weeks. I thought that now we were all paying monthly subscriptions for Creative Cloud that bug fixes were supposed to be rolled out quicker!
BTW The workaround that I have been using is to create MPEG2 Blu-ray instead of MPEG4. You need a higher bit rate for the same image quality but provided that the duration isn't too long & will fit on the disc then it's OK. Unfortunately this is not a satisfactory alternative if you have a two hour video.