I have now "seen the light" about how this has been designed, amended the outbound mqtt message from the controller to request ACK:
and this is working and sending back an echo message:

mygateway1-in/100/2/1/1/3 100
mygateway1-out/100/2/1/1/3 100

So I can now build a rule round this in openhab to handle errors. I suppose I should have thought this through before pestering the forum but then again that is what it's for! Thanks again.

@mfalkvidd Hi. Thank you. Most useful. Just tried and it reports:

I2C Scanner
Scanning...
I2C device found at address 0x40 !
done

So seems to be the same as si7021 and correct. Will respond to @Yveaux with more details as still not working with si7021 library.

@mfalkvidd thank you for this helpful response. I will respond once I get a chance to have a look at this properly again.