iphone 3GS, no service on 3G

If I enable 3G I get "No Service" but when turned to off I get a full EDGE signal, my father has the same phone as me and doesn't the the same problem, what is it likely to be?

Also to note I have just installed the very lastest update (4.3.3) however the problem started when I was on the old firmware (4.3.2)

My contract has ended and thought abotu just replacing it with something else, however I don't want an iPhone 4 as they will soon become obsilete, and I'm not paying £140 for an old phone when I can get a Samsung Galaxy S2 for free on the same contract. So I want to keep my 3GS untill the new iPhone comes out.

thanks for your time

Brett