I rang retentions direct - if you go to
www.saynoto0870.co.uk and search for o2, you'll find an 0800 number which takes you straight to retentions. I *THINK* that costs money to ring from your mobile, but all you need is a landline and it's lovely and free (suddenly you stop caring about being put on hold).
I was initially offered the contract at £30 a month, however here's roughly how it went, after I stated that I was looking to 'upgrade to a new phone and see if they could offer me a deal to rival vodafone's stop the clock' (for those who don't know, Voda's Stop The Clock allows you to make a phone call to any UK mob / landline in the evenings, the first 3 mins come out of your free minutes, then the next 57 are free. After that it starts to use free minutes again, but you can always redial if you wish)
He offered me either their Online 35 or Online 30 tariffs (go to the o2 shop website and take a look at the online texter 18 month tariffs), I liked the sound of Online 30.
Me:"well, that's a 50% increase on what i've been paying, and as a student that's quite a bit to afford; is there no way we can keep it at £20?"
Guy on phone: "Alright just a moment will see what I can do"
*frantic typing on his computer*
Guy on phone: "yee-eeep, that contract's now £20 a month instead, there's been an 18-month £10 a month discount applied".
Hope this is of some use!