Many thanks for further explanation and Catch-22 cautions.
I previously did an OTA update that succeeded (which included the incorrect APN as it happens), but luckily as you explained here this did not take the electron off-line as the device has not been powered down and the modem has not been hard reset and continues to use the previous (correct) APN.
Now I cannot perform further updates to correct the APN error (even though the device is working perfectly otherwise) and so should the device power-down, it will take it off-line!