This requires some haggling with three, i haggled few weeks ago and managed to get my self £13 a month deal unlimited text, minutes and 8gb which i assume was a retention deal.
Today i rang them up saying i want more data without breaking my wallet and other providers are doing great deals right now and i gave the example of virgin deal for £15 and that they offered it to me for £7.50 a month.
after hearing this the advisor gave me their usual deals they offer or have on sale but i rejected and said i do not want to pay more and simply want more data and straight away he said he will speak with his manager so after 2 minutes he offered me that deal unlimited EVERYTHING! for £16 a month on 30 day rolling contract and he said its beacuse i have been with three for over 3 years.
good luck guys but i guess it might work mainly for peoples who had £13 a month rentention deal with three and also do not expect this at first call as i had to call twice beacuse the first advisor was no was lettting me have a cheaper deal.