I have had a lot of help with synch but still have not solved the problem as it does not consist of syncing two audio tracks but of a timeline glitch causing the problem.
I have been told that the problem either came from the camera or from previous incorrect joining of clips and the original clips have vanished. The present problem occurred in the first 3 minutes of a video where the video completely vanished into lines. for a minute or so. I have been wondering whether I could first take the video before the fault into the timeline assuming that that was OK . Then taking the video after the faulty section I could perhaps put the audio on a separate track and then try to time it into the first section.
Would that work. I think the sound track keeps going but do not know how to use the faulty section for the fusion. Perhaps I should just try to do it manually?