Hi freddie, have a look inside the old case.
There might be a small chip that needs to be put over to the new case.
When that chip is near the car it will start at ignition, sort of an anti theft device.
If you have the old case, try bringing that near the ignition when using the new case n key and if it starts, you know there is still something that is needed from the old. Perhaps it's not transferable.
It's often a seperate little piece that can look like a small piece of broken plastic and slots into a small groove inside the case.
And a Glaedelig Jul to you too tas. 
(very thoughtful of you)
Master Freddie done the replacing I will ring him and see if he had any spare pieces 
I made the mistake of vacuuming my pond out, with my Saab key in my pocket.............I disassembled it, used a hair dryer to dry it out, but no joy..........
Fortunately I had purchased a spare key the previous winter............but that's a spendy bath I took.
This is what one looks like..........
