Hi Tim, Thanks and I do appreciate your time in responding.
I confirmed that the printer is COM4 (no other USB devices plugged in besides keyboard and mouse) and unplugging the printer removes COM4 from both the device manager and the devices tab under PIO Home.
No other programs are open (checked in task manager), so I cannot see anything obvious that may be accessing COM4. However, I also know that I don't know enough about this to say categorically that this is the case. I looked for any services running under the properties for COM4 in device manager but services didn't appear in the dropdown list.
I have even tried doing a complete uninstall of VSCode and the PlatformIO plugin, and then going through the installation steps again. I rebooted the computer after each step.
My next approach will be to reinstall Win 10, which isn't all that work, as this is a computer in my workshop which doesn't have much on it in the way of software and all my data is on a separate HDD. I tried a restore point but only one was listed and it failed to restore 🤦
I've just followed the install procedure on my home PC and I noticed that it behaved slightly differently to my workshop PC. The main thing that I notice is the PlatformIO icon appears on my home machine but not my workshop once, despite both appearing to install correctly.