Thanks PCNemy, I've been meaning to go back to stock for a while, and your work seemed like a good enough incentive.
Restarted to the bootloader, then started Fastboot on the phone to give the PC access to ADB.
Read the readme.txt in the download, and followed the instructions.
This brought me back to 2.2.1 (update1)
Same issues with limited APNs, so you may have to skip the account setup process and add yours manually.
System update downloads 2.2.1 (FRG83D) automatically, then asks if you want to install the update.
Immediately on rebooting, a second update is available for install, which installs and reboots automatically, the build number moves to FRG83G and the Android version is now 2.2.2.
All things working as expected so far - nice piece of AIO work.
[Edit: Added more detail.]
Edited by jango76, 24 February 2011 - 04:25 AM.