Volkswagen India Private Limited - VW POLO TDI Messed up after Timing Belt change

Background:
I have had a * year love affair with the POLO *.* TDI. Amazing car and an amazing journey. A few months ago I gave my car for service.
I generally get the service done in front of me for the last * years.
This time I decided to get some denting and painting work done along with the service. I was assured a week for both as the service included changing the timing belt. However this rolled into almost a month. I am still ok with that.

The Problem
When I started the car on cranking there was a big shudder in the engine and it went off. This shudder also happens when switching off. I immediately informed the concerned that this was not normal as I had never faced this issue ever.
They said they would need to do base setting. I brought the car back to the showroom and It was done. But the problem continued.
Apart from the shudder and few Misses on the initial start I see that the pickup has unusually increased and the mileage has dropped drastically. My city mileage has always been ** and the highway goes upto **. I do not get anything beyond ** anywhere anymore. City mileage generally averages now to **!!

I am assuming since the timing kit has been changed It should be opened up again to see any variations in fitting the kit. I had given the car back to them and they have been checking everything else including the injectors and the pump etc which were not opened up at all.
The concerned people have not been able to diagnose the problem and keep blaming one of the injectors that is working **%. Had that been the case I would have had the problem before as well. Why did this happen after the service?

My Request
I would like to request for someone from VW to see the car and have the car opened up to check if the timing kit has been put properly. I am seriously disillusioned with VW after * years of bliss!! I am really not interested in disposing of this car off and request if you can find a solution. I have already spent *.** Lacs and also taken an additional service coupon for next year. I am looking for a senior and the best VW Engineer to see the vehicle.