Well adding locks to the i2c object as suggested by @haukex works fine.

The volts/amps data looks fine and there has been no disruption to the lcd dislay, albeit for 24 hours or so. As a test I removed the locking on both scripts, and to my surprise there were virtually no faults. I saw one display line corruption, but the display did not freeze or become unreadable as it had done before, but there were intermittent data errors from the amps/voltage devices.

Perhaps a kernel update has substantially corrected the problem. I started using the time slot code in November 2018 and I am currently running Raspbian Stretch with 4.19.66+ on a Raspberry Pi Model B Plus Rev 1.2, so there will have been updates since November 2018.

After updating scripts on a second Raspberry Pi (Raspberry Pi 3 Model B Rev 1.2) also running Stretch, there has been a marked increase in stability. One script that previously had unexplained crashes has been stable for almost 24 hours and data errors have stopped. This Pi has three scripts accessing i2c devices on a regular basis.

When using flock to lock the filehandles, note that the module to use is

The BCM2835 module

does not work - you get this message

The other gotcha was that using the default i2c bus on pins 3 & 5 (gpio2/gpio3) resulted in this error

This is resolved by opening an alternate i2c bus. Edit the /boot/config.txt file and add this:

# activate an additional i2c interface on gpio 6(pin 31) & gpio 12(pin
+ 32)
dtoverlay=i2c-gpio,bus=3,i2c_gpio_sda=6,i2c_gpio_scl=12
[download]

You need to be root to edit the file (use sudo). Reboot to activate. You can use any unused pair of gpios, preferably avoiding any that are designated for other functions. The two gpio's used each require a pull-up resistor to 5 volts.

The additional i2c bus is likely i2c-3. Enter ls -l /dev/i2c* to see the device id's.

Thanks to everyone for your replies and of course to haukex for a successful conclusion.