3G iPhone locked on the 02 Network. Best way of Unlocking...

Hi All,

I Have an Old iPhone 3G I think. Model Number MB489B It is running on IOS 4.2.1(8C148) With Modem Firmware 05.15.04. It is locked to O2 UK, & I want to Unlock it for any SIM.

I have Jailbroken the phone & Installed Cydia via the latest version of Redsn0w 09.15b3

What is the best IPSW to use to Unlock it using Ultrasn0w? I have read up a bit & assume that I need to "Downgrade" the Modem Firmware before I can Unlock? Is this correct?

Can someone give me a few pointers...

Much appreciated