How can I unlock my HTC One V locked by 3 (UK Carrier, www.three.co.uk)? I have a HTC One V which is provided by 3 (www.three.co.uk) under contract.

I recently bought a Nexus 4 from Google Play because of the price drop, and I'm using my 3 SIM in the Nexus which is working fine.

I want to sell my old One V and know that I can do it a lot easier if the device is unlocked so it can be used on any network.

After looking around online, cellunlocker.net quotes me $14.99 for an unlock code and Three themselves quotes me £15. I've heard from some friends that they were able to get their phones unlocked for free, if so, how would I go about doing this?

I'd rather not fall victim to a marketing scam to get a code that I could get for free.
Your local independent phone repair shop or market trader will probably SIM-unlock your phone for around a fiver, or you can probably negotiate it as a free service if you're buying something else there. It's not quite free, but if you value your time at all, it's probably cheaper than faffing around trying to do it yourself.