A friend by e-mail told me that GT-SIM has changed its rates for SMS and GPRS (which were never billed till little time ago, if I'm not wrong) from ? 0.09 to ? 0.18 and ? 0.10/10KB to ? 0.18/10KB which is really a BIG difference compared to the old rates!
On their site they still write the old rates in the homepage and the new ones in the rates page...

but it seems they are billing at the new rates already...
Quite a bad move from them...