Galaxy S5 after System Update is asking for SIM Network Unlock PIN

I recently purchased a Galaxy S5 on http://amazon.co.uk . This was a well deserved upgrade from a galaxy S2. When I got the phone I powered it on with my Vodafone Spain SIM card and it worked perfectly. The day after I realized that there was a system update and I ran it. As soon as the update ended the phone asked for "SIM Network Unlock PIN" Is there any way to revert this update? is reverting the best option? Can I unlock the phone without buying an unlock code? I found this site http://unlockcode4u.com recommended by a friend and they seem capable of unlocking the phone, but I just expended £250 for the phone and the code is really out of my budget. Please help.