Failing that, do you have a little local phone shop? They could probably test it for you and then unlock if needed.
Having said that, when I was trying to unlock a phone for my mum the other week I came acroos loads of websites that give you the codes free to unlock various models. You could try that, it shouldn't harm the phone if its already unlocked.
I did have a load of O2 sims from when they did their free offer, they sent me 4 instead of 1. If I can find one you are welcome to it.