You need to be an existing O2 payg user, and have a spare brand new free sim.
1.Get new sim card.
2.Send an SMS from existing handset with existing SIM Card to 865555 with the Mobile Phone Number that came with your new SIM Card (e.g. 07999123456) as the message.
3.O2 will send back a message confirming your upgrade( but of course you have not upgraded just got a sim)and tell you your free calltime credit amount.
4.The calltime credit will be applied to your existing phones calltime balance in 14 days. The new SIM card and new mobile phone number will then be disconnected.You retain and use your existing sim,your existing number, calltime balance, Bolt Ons etc.
5.Once calltime credit has been applied, bin the new SIM Card
Basically its a loophole in the O2 payg upgrade system. Handy if you dont intend to upgrade in near future.
Dont say i'm not nice