I got it working!
I did the following:
1. Set my laptop's wired connection IP to 192.168.0.1; subnet mask as 255.255.255.0 and default gateway 192.168.0.1. Left DNS blank.
2. Opened Tftpd64 application and did the following:
* Went to DCHP tab
* Set IP pool start address to: 192.168.0.2
* Set size of pool to: 10
* Set default router to: 192.168.0.1
* Set mask to: 255.255.255.0
* Pressed "OK" button
* Selected 192.168.0.1 (ethernet connection) from the "Server interfaces" dropdown
3. I then connected the ethernet to ODB2 cable to the car and started the car engine.
4. I pressed and held down the radio power/volume button for 30 seconds
5. The iDrive screen restarted
6. I then noticed that the tftp32 application indicated that it had assigned 192.168.0.2 and 192.168.0.3
7. I visited
ftp://192.168.0.2/ in my web browser and was promoted for a username and password
8. I entered "root" and "Hm83stN)" and logged in successfully.
9. I then downloaded the file in question.
I'm now about to try getting the part I need using a hex editor. Will let you know how I get on.