Knackered my old 3GS (my son chucked it in the bath.) Jailbroken SHSH blobs back to 3.1.3....

Now have inherited 3GS on iOs 4.3.5 bb05.16.02 from mate who got free upgrade. Locked to O2 No SHSH blobs. What is my best option to jailbreak and unlock? (Prefer untethered).