Big thanks to Mark Webster who got in contact on Saturday with the below walkthrough on how to jailbreak 4.2.1 using greenpois0n.
From Mark: This may be interesting if you want to use it. After 10 hours of effing around I finally got my iphone and ipad JailBroken.
- Backup your devices on itunes.
- Restore to 4.2.1 on itunes.
- Download greenpois0n.
- If you’re having problems getting it to run then keep your finger on the home button the until the device reboots.
- GP puts ‘Loader’ on your device and is supposed to install Cydia but there are huge problems with that right now so the advice is to run redsn0w 0.9.7b6 and just check ‘install Cydia’
- There were more problems here – ‘unexpected error’ kept coming up when it went to install. The very infuriating and simple trick was to restart the Mac and run redsn0w then. It worked first time after that.