He removed the whole roof lining from the car and saw the water appear in the car from between the roof and the car.
He removed the whole sliding roof assembly from the car, and found almost no seal between the car and the roof. The remaining seal was damaged and worn.
Eddie used a silicone-gun and sealed the whole roof again. Then placed the roof back on top of the car.
At the back of the sunroof there are 2 drain holes and tubes. They were closed with dirt, he cleaned them with compressed air (don't apply too much pressure or they may blow off inside the car!). Fill the roof with some water and it should come out just behind the rear tyres.
Easy way to clear these tubes is to remove the rear light assemblies and pull the tubes up to expose the ends - there is enough slack to mouth-blow back up into the roof assembly.
When the headliner was out Eddie cleaned it with some alcohol vinegar, which took out the yellow marks form the leaking roof.
the sliding roof assembly completely removed after removing the interior trim and undoing the fixing bolts.