Then is the 'engineer' an employee of the Opel garage, or an independent assessor? I stand over my initial view that if insurer is aware that their assessor also owns a garage, then he is not 'independent'.
You should get your own assessor to see the car. At least with his report, you will have some documentation to argue/debate with insurer.
On the other hand, was the 'joyrider' caught/identified? If he was simply an uninsured driver, as opposed to a 'joyrider', by which term I understand to be a car thief, then the insurer of the offending car may be involved, rather than MIBI. That being the case, when you add on depreciation to your car following repairs, that insurer may be willing to give you a new car.