Hey,

We have just got an Iphone 4 off my mum but it's locked to orange and we could do with the phone being functional straight away. Need to unlock it fr use on o2. Does anyone know a way we can do this tonight?

Apart from a jailbreak (which if it's on the latest software you wont do easily) it can't be done tonight.
The cheapest way, if someone has been on Orange for a while, is to get Orange to do it, will cost you about £20 and take a couple of weeks.
Shops will be able to put it through for you, but they'll charge about £100 for Orange, and still takes a couple of weeks.

Have a look around Birkenhead Market, there are phone stores in there that will unlock it for you for a fee. It takes about an hour.

http://www.iclarified.com/unlock/

Easy to follow guide, as dr says though. Not very easily done on newer OS 7