ive put a giffgaff sim card in my iphone 4 and its saying no service

I seem to recall legal issues with use of that sim in many countries - try dealing with the people you got it from, or try a valid sim from a service provider.

Giffgaff is OK. You are thinking of a different SIM that I won't mention by name.

As Lawrence has said GiffGaff is actually a UK Apple approved Carrier

Is your iPhone locked to another Carrier ?

Right it must be that other one that is not to be named...now that is an idea for a book theme, hope it hasn't been used yet 🙂