In message , druck 
writes
>On 01/02/2019 16:40, Adrian wrote:
>
>[Snip]
>
>> So, does that suggest that the bus is working, or that that I've got
>>the right bus, but there is no data or what ?
>
>It looks like the internal i2c bus 0 is enabled, and not the external
>bus 1, which your sensor will be connected to.
>
>Try going back to a stock config.txt and cmdline.txt from a Raspbian
>image, reboot, use raspi-config to enable i2c, reboot again.
>
>---druck

Thanks.

I've done that, and now I get /dev/i2c-1 present (but not 0 !)

Running the python code, I now get readings from the sensors.

Problem resolved.  Many thanks to those who've contributed.

Adrian
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