Peugeot 206 2.0HDi new key fob immobiliser fault?

I have bought a new 'case' for my 2002 206HDi car key. I have swapped the case only (button pads had fallen out), and kept the circuit board & the metal key itself. BUT when I insert the key in the ignition the dash displays "immobiliser fault".
If I swap the parts back into the original casing, then the ignition responds OK. I assume there is some sort of "ID chip" in one half of the casing that is 'read' by the car column ignition area?? How do I correctly repair / replace the key fob please?