Something doesn't add up here, I presume the car was driving/behaving perfectly prior to the major service work a week or so prior. I'm no mechanic, but timing chain stretch on a car with such low miles, would have to be extremely rare. I'd be expecting logical explanations from the garage. To me it sounds more like technician negligence, the mechanic working on your car hasn't paid enough attention to the detail to his/her work on your car, maybe hasn't fully tightened certain nuts and bolts, or maybe dropped or left a third party object in the engine, forgetting to remove it. The list could go on.
As mentioned above, I would be asking an independent engineer as to why this occurred without any warning lights or any significant indications.
Personally, I'd be chatting to my solicitor too, as the dealer's behaviour to just point the finger of blame solely upon you would be making my blood boil.
The best of luck to you, I hope this matter get's resolved promptly in your favour.