I have prepared a set of log files, which I will upload shortly. In the meantime, I think I have identified what the problem is, although I don't understand the mechanism.
The Walkman was reported as full, so as an experiment, I deleted an album from the device and then tried synchronising. This time, MM4 produced a list of files to synchronise to use up the space that I had freed. I then tried synchronising again, and this time MM4 give me a list of tracks to delete in order to create room for the new tracks, as it should do, but only using tracks in the space I had originally created, and does not look at the rest of the tracks on the player. I had problems synchronising with MM4 the first time, so resorted to wiping the device and then Synchronising. MM4 does not seem to regard these tracks as available for future synchronisation.
I am going to delete all the tracks from the device again and see whether a subsequent Synchronisation will cure the problem. Hopefully the logs might identify the actual problem.