I'm pretty sure the PUC (PUK?) codes won't help .They're not used to unlock a phone that is 'locked to a network', but to unlock a phone that is just 'locked' - i.e. been messed around with to much, wrong passwords entered etc until it won't allow anyone to use it at all, whatever the network.
Anyway - if it really is only a network lock you're trying to overcome then
this guy on eBay will charge you £2.70 and provide you with a code that you can enter to unlock the phone. I think it will be based on your IMEI number which I expect he will tell you how to extract from your phone (possibly dial *#06# this certainly works with all Nokias, Ericssons and LGs I've owned). I've had to use a similar service on eBay for a different phone and it was all very easy and pain free.
Also see
http://www.uniquephones.com/unlock.php Maybe a bit more user friendly.
I have tried to find a website for you that calculates your unlock code for free but can't find it at the mo'.
Good luck.
(and of course a disclaimer: I bear no responsibility should you lose £2.70 or damage the phone while trying this method etc!)