Well, I think the problem has now been solved! At the suggestion of the dac manufacturer, I have inserted a Fast Ethernet Switch between the dac and the router to force up the Ethernet speed. This seems to have cured the problem, everything now working as it normally did. He thought that maybe the latest Roon update meant that my NAS is now too slow. His theory looks like he was right.
Thanks for your efforts in helping me, but it may be that you should look into this as a possible solution for some other problems elsewhere