Yes, the latch in the car does not return to its normal position. So it looks like it is a cable problem. My (one-man band, but very experienced) mechanic says he has never come across a problem like it. First time I have ever known him baffled. Looks like a trip to Lexus to in Hatfield for this one.
Thanks for the various suggestions made so far.
Well - a happy ending!! 3 days after my bonnet-lock problem first appeared, preventing me from putting the bonnet into its locked position, I have just been back to the GS300 to apply some WD40 to the bonnet-lock to see whether that helps. VERY surprised to find that the release latch in the car had mysteriously returned to its normal position - which it was refusing to do before and just hanging like a broken arm. I pulled it in the normal way and, incredibly, the bonnet is releasing and closing as normal.
I also notice that when I (un)lock the car with the fob, the lights on the sides of the car have begun to flash normally (were not doing that last night).
I still wonder whether there is some electrical factor involved in all this - given that all this started after I had put the battery on charge without disconnecting the terminals (??)
My Lexus has been so reliable (since 2000) that I have rarely had to open the bonnet, except now and again to charge the battery. But I think I will be opening it more regularly now to make sure the cable mechanism gets a bit more reguar exercise.
