Suddenly a quick solution thanks to blutach! Just after my reply I realised what PUOps are - this is an acronym for 'Prohibited User Operations'. I checked PUOps for the special features menu in DVDRemake and found that somehow a number of items were ticked. I cleared all these and now both title and root menu buttons are enabled. Ah, knowledge is power. It would never have occured to me to think of PUOps as a possible cause of my problem.

How nice it is to find a user group that provides such good advice.