Yes Pert, you are quite right, but you have the advantage of knowing a lot more about the subject than me.
When things go wrong for no obvious reason, I generally go to the web for advice and the only item I found on the CH340 not behaving advised looking in Device Manager>Other devices and if the chip had been detected, it would show as a serial (generic) device. If the driver had installed, it would show as CH340, but still in Other devices. Luckily, I found it in the Ports section. Other users of CH340 have reported problems with the driver, but these clones had worked out of the box and I didn't need to install the driver.
I can throw a bit more light on this problem. I use two PCs for Arduino work. One runs IDE 1.8.3, the other 1.8.5.
The Nano worked on 1.8.3 and not on 1.8.5, but, yes, I had updated the AVR boards package to 1.6.21 the same day on one PC, but not the other, so one showed two processor options for the Nano (ATmega328P and ATmega168P), and the updated one showed the extra ATmega328P(old bootloader).
I think I must have accidentally changed it as I don't generally go any further than select the board "Nano" and don't touch Processor choices unless I have reason to do so.
I'm still none the wiser on the significance of "old bootloader" in relation to this clone with CH340 and without a "genuine" Nano with FTDI to test it against, I wouldn't know which one to choose. So processor choice ATmega328P works on one PC (the non-updated), but not on the updated one with the extra "old" choice
The good thing is that what was becoming a search to find non-working USB ports (Win 10 new settings was showing FT232 error) is now resolved and another lesson learned. I will see if there is any information with the update that I should have checked, but it's a case of ticking accept without going through reams of T&Cs.
Thanks for your input