I was pointing out, mainly, that switching to Tesco, which uses the O2 network, will save you money..:-)
It also offers an excellent range of mobile phones at prices that will almost certainly suit your budget. Your current SIM will more than likely work although you may have to reconfigure the settings in some cases.
If necessary, Tesco offers advice on how to unlock phones on its website (Virgin Mobile offers similar advice with its new SIMS).
laurie53, what bit about bying an "unlocked" phone and sticking your simm in it do you not understand? You just buy a phone that is not locked to any one provider, and stick your simm in it and it will work.