I think tesco mobile phones are linked with o2 if so will a o2 branded phone work with a tesco mobile sim card ?

yeah should do. My giffgaff works in o2 phones.

Is giffgaff on o2?

yeah! Your sim will work in a o2 phone.

Giffgaff wont work on tesco mobile I have got a giffgaff sim and have just tried it and its a no go so i'm guessing even though there the same network they wont work on each others network even though there the same providers.

tesco phones are locked to tesco.but tesco and gg work in o2 phones.

Thanks need to pop somewhere to test that theory I am hoping your right though

it will work. Tho why u on tesco tho?

It does depend on the phone, although they are on the same network there are several "levels" of locking a phone to a network, although you are right GiffGaff and Tesco all use O2, the same as ASDA uses Voda.

Which phone is it you have, might be a simple unlock.

Im looking for a xmas present the payg ones from tesco are rubbish and the sim free/unlocked ones are out of my price range.
So I thought o2 payg might work with a tesco sim as they seem to offer a better selection of payg phones.