Ive fixed the boot leak on our 06 Plate Focus Hatchback.
The boot was soaked on the right hand side, the boot lining was damp etc and droplets of water showing on and under the right side parcel shelf plastics.
Where the top of the torch is, all this was damp and lifting the boot carpet was damp underneath too. This had led to a lot of condensation on the windows of the car.
Looking up inside (where the arrow is) there were drips inside here, obviously it was coming in from above...

My first thought was it was the light cluster, but a haynes manual and removal of the cluster (just to see how it was cabled etc) revealed it wasnt that.
So, plan B. After heavy rain, I would nip out at night with a torch and must have looked like a right nutter to the neighbours, but I caught some drips making their way down the outside of the white trim and onto the plastic shelf, then on downthrough the gaps and down to the boot:

Anyway, I removed the tailgate inner top trim cover (it pulls off) This reveals four bolts that hold the spoiler on, recessed into the tailgate. They are approx where the arrows are, behind the trim.

Another night of rain and the culprit was two of the bolts holding the spoiler on, sitting there with drips forming, ready to run down the cover, if it had been in place.
Undid the 4 bolts, I removed the spoiler, (you also have to unclip the high level brake light), throughly cleaned up the seals on the spoiler that are supposed to prevent water ingress, (they are like foam, stuck to the spoiler) cleaned up the tailgate area where the spoiler sits and these foam seals mate, and refitted it. The bolts were tightned up firmly.
Then I decided to silicon sealant over the two tightned bolts that were leaking, as a double measure.
Its been dry since.
Hope this helps someone else, that has a tailgate leak.