I am most grateful to a thread on AVForums for the fix:-
There is an easy solution to fix this problem:
-With the TV in STAND-BY Mode press the following keys in sequence in the remote: Info, Menu, Mute and Power on the TV.
- The TV shows the service menu, find the menu OPTION, press Enter> Factory Reset, scroll down to MODEL press Enter then Right or Left buttons to see the various model numbers until you get to UD5800. Press POWER button. See images below
And voilà! Yo can play videosTurn TV on, change Source to USB and you should now have a Video option
I am delighted to say it worked for me at the second try. I think you need to press the buttons fairly quickly for it to work.
Here are some shots of the set-up screen that opens when you follow the above instructions.
This is the first screen to appear
PRESS OK for Option
Scroll down till you get to Model
Use L and R buttons to search for UD5800
Press POWER button