Exhaust emmision lamp and Juddering

[Max Golf]

I have had my 2003 Golf 1.6 petrol since June. Has been running fine until it got cold over the last few wks and have been struggling to get past 30 MPG of fuel, (I get 46 MPG when its warmer.) So had Oil and filter changed a couple wks ago, and new front tyres as they needed doing anyway. This didnt make much difference. Shortly afterwards, the 2000 mile service indicator started.
So i booked the car into VW dealership yesterday asking them to have a look at fuel consumption and general checkup. Girl from service dept called back to say tests show that nothing is wrong and that fuel consumption increase in cold weather is normal, and offered to do "Emmision Control Service" which i agreed to. I subsequently realized this was just a liquid they put into the engine that cleans and improved performance.
Now the problem starts: after paying for the service and retrieving the car from the forecourt, i tried starting the engine as normal, but it stalled straight away. A few more attempts were no use so thinking it was the cold (it was -1 degC) i managed to get it going with a couple of jumps on the pedal. Then as I pulled out of the forecourt to join one-way rush hour traffic, the engine suddenly nearly stalled again, the "exhaust emmision" warning lamp came on, and the "Emmisions Workshop" message flashed up!?! Unable to stop in the traffic for another mile or so, i struggled on with a juddering engine, lack of power, and warning lamp, before i pulled over and called the VW garage to explain. They went straight on the defensive saying that the treatment couldn't have caused that but that i could bring the car back tomorrow if there is a free slot.

[Max Golf]

Update:
Managed to judder all the way back to the VW garage in the heavy snow this morning and had the problem diagnosed as a coil pack gone bust causing misfiring. Apparently after one gets changed, the others start going. As i have a lot of driving to do this wkend, i decided to have all four changed for new ones. The damage - £200. But happy that car is back to its usual self.

Any comments anyone?