Edge settings wont stick...Delete pref file???

[danl7562]

I recently switched from O2 PAYG to O2 simplicity and while i can change the preferences and get edge to work when i restart the phone they go back to the O2 PAYG settings i had. I have tried the Reset network settings option but it hasn't worked.

Is there a preference file on the iphone that i can delete to solve my problem???

All help appreciated, Thanks!