The car has been back to the dealer 4 times. They have replaced the rear shock absorbers but the problem didn't go away. They have replaced the striker plate for the hatchback and this made the problem worse. Now they say that they can't hear any noises and I should basically stop wasting their time.
The striker plate is completely non-adjustable as it is held in place by two countersunk headed bolts which pull it into the same postion every time. Purely designed for easy build not effective maintenance. This silly problem is completely putting me off the car.