After a frustrating few hours, I found this post http://forum.xbmc.org/showthread.php?p=7...post739034
Which gave great screenshots and simple instructions on getting the apple TV into DFU mode. Turns out I'd actually done it with the first USB cable I tried.
OK, so now for the problem. According to windows device manager I have "Apple Mobile Device (DFU Mode)" with a yellow warnign triangle.
"The drivers for this device are not installed. (Code 28)
There is no driver selected for the device information set or element.
To find a driver for this device, click Update Driver."
Now, when I run Greenpois0n I click the prepare to jailbreak and lett it do the countdowns (as the device is already in DFU mode) but it just goes through and asks me to "try again"
Any ideas please?