First of all Cypherus thanks very much for your reply, I'm sorry that it's taken me so long to reply to you. I didn't follow your adventurous link however I did download the VLC player. Unfortunately no joy and much head scratching.
I have just had occasion to move my computer which involved unplugging everything and re-plugging in. On doing so the icon informing me that the ethernet controller was unplugged had disappeared and everything was working just as it had before.
I am still no wiser but at least everything works as before.