I had a look today and yeah Jay you were right, the black plastic thing that runs from the rear light cluster to the rear door does carry the cables to the number plate lights.
The orange/blue cable has a nick in it.
The black cable has been completely severed in two places.
I shall try to rewire both cables from the plug behind the light cluster to the number plate wiring tomorrow.
It was minus 5 out there today, (real brass monkey stuff) so wish me luck.
Will report back as soon as possible.