i have found a kind of loop hole in the T-Mobile contract system.
normally on sim only for £15 a month you would get 350 mins, 300 texts and no internet. but t mobile do these things called 'flexible boosters' and one is included in every contract so you could choose the unlimited internet one.
But, for £5 a month extra you get an extra booster. So, if you buy the £10 30 day contract you get 100 min, 100 texts and no internet, but if you add the unlimited text booster and unlimited internet if will only cost you £5 extra.
So, for £15 a month on a 30 day rolling contract you get 100 mins, unlimited text and unlimited internet (fair usage of 1gb normally or 3gb if you have an android phone)
Hopefully i've explained that well enough :)