Ok for all those who are wondering I have just called up O2 and they said you can just give them a call once you have your PAC code and they will transfer the number over! Yipee!
As long as it's within 14 days though...
Good to know, I've ordered mine from Vodafone but doubt it'll arrive by Friday for the activation. The codes are valid for 30 days from the original carrier though.
I'm still waiting for my PAC code from Orange (who promised I'd have it by now) and was wondering if anyone knows if I'll be able to register my handset and then change my number once I've got my PAC code?