Hi Kieron,
The antenna on these things is bizarrely the back of the casing from what i could see when i dismantled one of these a few months ago.
If you look at the pictures in step 13 of the tear down guide:
[guide|3641|page=2]
The copper plated edges around the casing make contact with the copper plated PCB on the motherboard.
As with most phone antenna's they have an inductor, generally to reduce call noise, but if it falls off or becomes damaged it will break the circuit and you will not get barely any signal, check if its still in tact.
In Step 15 you can see it to the bottom left of the turquoise outlined chip