I'm very happy to report back that my BMW Motorrad Navigator V is fully operational again and I can't thank you enough Butters.

Just in case anyone else goes through this I'll provide some info below:

I formatted the BMW Motorrad Navigator V 'Internal Storage' instead of the SD Card using the Garmin support article on my MAC which formatted it to the wrong settings.
This meant that when the device was connected to my Windows10 computer it registered in the Device Manager as 'MTP USB Device' under Portable Devices.

I put the SatNav into Mass Storage mode as follows:
- At the 'Where To' screen press Volume.
- Press and hold top right of display for 5-7 seconds until menu appears.
- Scroll down to MTP Settings.
- Select Mass Storage and Save.

I removed the SD Card from the device and used RMPrepUSB to re-format the Internal Storage using the following settings:
- Partition Size: MAX
- Volume Label: GARMIN
- MS-DOS bootable [IO.SYS] (CC3 and CC4DOSUFD)
- FAT32
- BOOT as FDD (A: no MBR)
Nothing else was selected/ticked.

After formatting I disconnected / reconnected SatNav and it registered in the Device Manager as 'BMW Motorrad Navigator V' instead of 'MTP USB Device' so I guess that is the indicator that you have formatted the Internal Storage correctly. However it still wasn't recognised by Garmin Express. I think at this point I tried a factory reset but I'm not sure I needed to but this is done as follows:
- Turn device off.
- Press and hold bottom right of display and turn device on. 'System' will appear on the screen but keep holding down until you see 'Loading'.

Device still wasn't recognised by Garmin Express but when I connected the device to my computer the computer image was orange. I disconnected device and restarted and then set the device to Mass Storage again as above and then reconnected to computer and now the computer image on the device was white.

When I loaded Garmin Express it recognised my device. I then loaded the United Kingdom Map and tested the device - worked fine.

I then formatted my SD Card (I did it outside of my device so I didn't mess up again but you should be able to do this with the SD Card in the device probably in Mass Storage Mode again. I formatted the SD Card using RMPrepUSB again but slightly differently from the Internal Storage:
- Partition Size: MAX
- Volume Label: SD_Card
- MS-DOS bootable [IO.SYS] (CC3 and CC4DOSUFD)
- FAT32
Nothing else was selected/ticked.

At this point I done another factory reset to make sure the device was back to defaults and connected to Garmin Express and installed maps 'Full Coverage of Europe 2020.10'.
After Maps installed I clicked on 'Tools & Content' in Garmin Express and clicked Utilities TAB, Then Language Files and installed what I needed. Then I clicked on 'Other Files' and installed everything I required.

Everything back and working.