BINGO!!
(So I had to go on to JustAnswers.com & pay £28, but I got the answer straight away ... wish I had gone earlier!)
Happy to share, though, for anyone struggling with this annoying problem...
I went on to Device Manager (Start Button > right click on Computer and selectManage) and uninstalled the device calledIntel(R) High Definition Audio HDMI. Restarted the computer. Connected TV with HDMI cable. Right clicked on the little speaker symbol & slected Playback Devices. Panasonic TV shows as a device. Right clicked on it & set as default. I got my sound back & everything works fine.
It seems the driver had been updated somewhere along the line and was no longer compatible. Following this process makes the computer put back its own driver. All this time I was looking for a newer driver, and what it wanted was the old one!
For the record, the device in Device Manager is now called High Definition Audio Device. No mention of HDMI!
By the way, if you right click on the desktop you get a wizard to set up the TV interface.