Free St Patrick's Day Ringtone

Hi there...not sure exactly what I did wrong, but I clicked onto the 'Free Stuff' website and scrolled down to Free St Patrick's Day Ringtone. All I did was enter my mobile no. and submit after selecting the tone I wanted. Next thing I received 2 x messages and £4.50 disappeared from my phone. I am horrified. The first message advised me to text a certain number tomorrow and I would receive my 'Free St Patrick's Day Ringtone' and the 2nd message stated that I should text 'Stop' to the previously quoted no. to stop my subsciption. I am mortified as I don't usually fall for these scams.....anyone else heard of this and I am now concerned that when I put more credit on my phone am I in danger of it being creamed off again???? Will it be ok or should I buy a new SIM card? I am trying to save money not waste it, really annoying...especially as I thought Free Stiff website was Kosher.

Any comments or suggestions would be welcome, thanks.