As i understand it you are on a rolling contract at your existing tariff - which in a nutshell is a lot more than you need to pay.
You should check with O2 and their SIM only deals. You usually pay a fiver a month more for a rolling contract than for a 12 month deal.
If you are happy with your phone and don't want to upgrade, personally i would get a PAC code from O2 and switch to T-Mobile. I'm on a £10 a month tariff with 600mins, 500texts, and 3Gb data.
A slightly different deal is available here; http://www.hotukdeals.com...er-month-tmobil/896115
£7.50 per month, 600mins, 500texts, 500mb data, plus a booster. The booster lets you choose an upgrade each month, which personally i would go for either the unlimited text or 3gb mobile internet upgrade.
You are only tied in for 12months, paying the cheapest price you can get for this sort of deal.
Well worth considering.