How can I ask for my money back

How can I ask for my money back when an App. is advertised as free then totry and use they charged £2-50 then when you try to use it again they ask for £13-99 this is sale under false pretences, I paid the £2-50 thinking that was it but no they just keep saying they want more and more, so when you stop at £2-50 the App is totally unusable and I mean totally unusable you have lost £2-50