Unlock my Samsung Mega, but where/how?

I recently traded my Note2 for a Mega, but it turns out it isnt unlocked.

I have tried a couple of downloads to unlock direct unsuccessfully, but i have never been much interested in phones so general knowledge on them is very limited.

Can someone recommend a reputable unlocking firm I can use? It is locked to O2 and long out of contract, original user had it on giffgaff.

Local phone guy quoted me £30 and at least a week, was hoping for something cheaper and faster..

A work colleague has an O2 contract so has requested an unlock on it FOC.

At worst It appears I could have used a PAYG sim to get O2 to do it for £15, seems the only way.

Can't you just buy an unlock code from eBay?

Ebay will be fine for this.

Tried and failed on 2 so far, received a refund each time...

Recommended one directly on another forum which I am trying, but I am sceptical.