Problem solved....    :)     (I hope.)

> Steve Lawther replied:
> Try putting a pull down resistor (100K) from the pin to ground,
> or across the LED, as the I/P is floating - worse, the LED may
> be generating a (very very tiny) current from light. (I believe it can
> happen)

I've only done a few quick tests but it appears to solve it.  With the
additional resistor it works correctly.  Without it, I can trigger it with
a flashlight!  Also, never noticed the problem occurring during the night -
always during the day.

> I use the inverse (LED to vcc, push button low) of your circuit (with
> the extra resistor) on my graphic LCD checker:-

Is there any particular advantage to either arrangement?

Thanks.....Dave Scott